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Tecnifibre Black Code 4S String Review

The string for maximum spin, how does Technifibre's co-poly perform?

Designed for big hitters who want a durable, low-powered co-poly with plenty of spin potential and impressive tension maintenance, Black Code 4S is a co-polyester tennis string from French manufacturer Tecnifibre. It features a square shape with four spin-friendly edges. 

Along with the square design, Technifibre uses Thermocore Technology, a multi-stage heating process designed to soften the structure and reduce harsh impact vibrations. 

So does Black Code live up to its billing as the best string for spin? Find out in this Tecnifibre Black Code 4S review.

Tecnifibre Black Code 4S Specification

Technifibre Black Code 4s
  • Monofilament polyester
  • Square cross-section
  • Gauge tested: 1.30 mm / 16 G
  • Stiffness: ~209lb/in (or 3.74kgf/mm). This is considered “stiff”.

Racquet Setup For Testing

Strung at 24 kgf on a Babolat Pure Drive 2015, in use for about three months now. It's a stiff frame but with a large and tolerant sweet spot.

Due to its inherently high efficiency (too high for some) in transferring most of the player's power to the ball, a polyester string fits this frame very well to tame all that juice.

First Considerations

Technifibre 4s First Thoughts

Check Latest Black Code 4S Price

Tecnifibre is a French company whose reputation has been built mainly on the quality of the strings they produce, and there seems to be quite a large number of ATP professionals using them.

“4S” is the second version of their popular Black Code polyester string. The original was a 5-sided cross-section string; this one is square.

Virtually all manufacturers produce one or several non-circular cross-section polyesters, together with the corresponding claim that this shape or the other will help you with the spin, control, ball bite, etc. Of course, much of that is exaggerated, but there is some truth behind it.

Polyester strings help with the spin primarily because their low surface friction allows them to slide and snap back easily when you hit the ball with the racquet face tilted.

On a square shape string such as Black Code 4S (and many more), that sliding action is made even easier because a string will sit on a cross string on one of its four faces (and go underneath the next one by the opposite face).

This also has the advantage of decreasing the chances of notching because it's mostly faces against faces; if the number of sides were odd, notching would occur in all crossings.

How Does Technifibre Black Code Feel and Play?

Technifibre 4s Square Design

All the talk above about cross-sections and sliding translates in practical terms in a very curious manner: Black Code 4S, like most polyester strings, is very firm on flat shots but much less so when you put some spin.

I would put it around the middle of the scale, though I don't have many more references. Due to its low elasticity, they will return a smaller fraction of the energy you put on the stroke when compared to nylon.

The immediate impression is a substantial and direct impact, which can get just a little uncomfortable at harder strokes.

However, it never gets jarring because Black Code 4S dampens the initial shock quite well; I couldn't feel any annoying after-shock vibrations on a stiff frame as the Pure Drive says a lot.

The response of 4S is very predictable, which helps a lot with directional control. Very seldom, I found my shots sailing long. This is a control-orientated string even if you tend to hit flatter balls, and it invites you to swing away (beware of not asking too much from your muscles and tendons).

I like polyester strings because they sort of “compress” your input: if you swing faster, the balls will not go faster in the same proportion. Especially if you hit with a bit of spin, a faster swing will transfer more and more energy to ball rotations and not so much to linear speed. This is quite beneficial for control and is a great confidence inspirer. But this also requires a lot of physical commitment.

Now, the exciting bit: if you hit with spin, these strings move sideways very easily (try to push the mains sideways with your fingers, and you will see immediately what I mean), so a top-spin or slice groundstroke will feel much softer (relatively, that is…) than a flat stroke.

The impression is even slightly muffled, which can get disconcerting at times. Whereas there was already excellent control on flatter strokes, the amount of spin available amplifies that to a point where it's almost (so to speak) impossible to make the shots sail long. This gives you the ability to cut very nasty slices that will bounce low or exit sideways if you put a little side-spin on the ball.

This spin capacity remains unaltered for an unusually long period of use because the notching is near non-existent: remember that most of the string-to-string contact is between flat faces, so there are no sharp edges to cut a groove on the neighbour. Since I don't hit so hard, I did not notice tension loss yet; if there has been some, nothing compromises my game.

All this control works to your advantage only as long as you have fluid swings and assertive strokes. It's useless to place the ball where you want if it arrives there dead, just waiting to be attacked back and returned to you as a winner. A little less than total commitment won't work. This is why beginners might be a little disappointed with such a firm string.

This is especially notorious on the serve since it's the stroke where you must provide *all* the power. Black Code 4S will give you nothing in return if you feed it with little energy. It would be best to keep the service motion fast and, very importantly, with a loose arm, especially on the slice and kick services that require a lot of racquet head speed.

So, if you're like me, you'd better hit the service a little flatter. Or move to nylon! I'm significantly penalised here because I tend to exaggerate the service spin with not much muscle to help to do that properly, so by doing that, I lose a lot of ball speed.

The experience is a little different on volleys: since the ball arrives faster, there is already plenty of energy to be deflected back as you please. I found Black Code 4S's firmness to work well because it makes it very easy to absorb most of that speed and redirect the ball at will with no traces that the frame is about to lose the composure, provided you keep a steady grip.

However, drop shots and other crafty manoeuvres are a bit trickier to perform because of the low feedback from these strings at slower strokes, so you must rely on your eyes more than on your hands.

There is a downside with Black Code 4S, which on the other hand, is not that surprising on a low elasticity string; the drop in power return is quite steep as you hit closer to the frame, and the experience can get uncomfortable. There is little tolerance here, so please work on finding the sweet spot as often as you can.

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Final Thoughts

Technifibre 4s String Close

Tecnifibre Black Code 4S is a very firm and control type of string with all the spin you need at your disposal but with little give or elasticity. The impact is more on the muted side rather than “metallic”, and it's not uncomfortable.

Personally, this is as stiff as I would go. It's not the type of string I would suggest to someone learning the basic strokes or in the early stages of game development. I would say that this is very comparable to a sports car engine: at low speed, it's helpless; step on it and gifts shall be given. But you need to be focused on what you do.

I liked

  • Solid and firm feedback with just enough dampening
  • Durability
  • No string displacement
  • Control
  • Spin

I didn't like

  • Slight lack of feel and pop on slower strokes
  • A little low power return
  • Intolerance to less than adequate technique
  • Loss of power as you add spin

A note about the scores: low power return is not necessarily bad, so it's a misleading metric. There are few things more annoying than doing everything “right” only to watch the ball launch off the string bed like a Jumping Jack with no sense of direction.

Tecnifibre Black Code 4S Review

Power - 5
Control - 9
Comfort - 6
Touch / Feedback - 5
Spin - 9
String Movement - 9
Durability - 8


Firm and Control Orientated

Tecnifibre Black Code 4S is a very firm and control type of string with all the spin you need at your disposal but with little give or elasticity. The impact is more on the muted side rather than “metallic” and it’s not uncomfortable. It’s not the type of string I would suggest to someone learning the basic strokes or in the early stages of game development. Personally, this is as stiff as I would go. I would say that this is very comparable to a sports car engine: at low speed it’s helpless; step on it and gifts shall be given. But you need to be focused on what you do.

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User Rating: 2.99 ( 21 votes)


Live in Lisbon, Portugal and work as a chemical engineer. Incidentally some of the materials my company produces are used for tennis gear, such as ball felts and carbon fibre for frames. I'm tennis passionate for as long as I remember and a huge Federer fan but generally of any players or teams who place sports ethics above winning greed. I started playing very late and practice twice a week and occasionally at weekends. I pay great attention to gear, which can sometimes get a bit too distracting.

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  1. I know this is my blog and I am biased but I enjoy reading these string reviews 😆 they are a very good primer before forking out money on strings if you are unsure of what to expect. I know not all readers play but I hope some find it useful.

    1. Very useful for all of us, Thanks Jon and Rui for all the great information on equipment. Strings are very important an most of us dont really pay attention to them.


    1. Probably because I changed some cache settings. I remember had that problem before but forgot what the fix was. Will try sort.

  2. It’s “designed for big hitters” so it means definitely not for me and a bit technical to appreciate fully for my level of the game 😆 but always good to read a well-written post. You must been also a good tennis player, Rui!

      1. I don’t have much game to forget about 😝
        My coach keeps telling me to prepare sooner and move to the ball instead of hitting from the back foot. Being the old daddy in the class does not help much…

  3. Hi Rui,

    I have read both your reviews. Which string do you prefer out of Black Code and the Dunlop Black Widow? Are they the two you use most?

    1. Hi. Thanks for reading. I prefer Black Widow, definitely. It’s possibly as close to nylon as you can expect from polyester. It’s softer, more elastic, more comfortable, looks menacing in its shiny black and makes a louder noise! (Haha, I like to hear the pop). Plus I tried BW on 7 different frames… from 270 g to 330 g and it was funny in all of them.
      Black Code feels deader, stiffer and it’s a little beyond my level because I can’t generate enough racquet head speed to take full advantage of its spin potential. I’ll keep it just for a few more weeks.

      1. Thanks. I will ask the stringer if he has some as I want to test. We just use Prince synthetic gut.

      2. Hi. That one seems to be very highly regarded. I have a similar string (Dunlop S-gut) currently on 4 different frames, from 275 to 310 g (a “beginner”, two “tweeners” and a “player”).
        This type of strings (single core nylon plus some outer wraps) is absolutely a no-brainer: string it at 25 kgf (55 lbf) and it’s ready to go, does not matter what racquet you put it on. Dirt cheap, comfortable, not too elastic, reasonable feedback, good durability, not spectacular nor bad in any quality.
        It’s what the doctor recommends to cure the obsession with trying different strings…

    1. No, I haven’t. (Sorry, it has been a few days since I last checked the post comments)
      I got curious about it when it came out and read a few reviews. From what I gathered it seems to be a sort of “starter” polyester, similar to Luxilon Adrenaline, which I tried on 2 different racquets, so I’ll base my speculations on this one.
      I would expect it to be firm and crisp as this material usually is and I certainly would not count on it to help much in power department. It’s round and as such will not give great assistance in spin either. However it’s possibly softer than most polyesters usually are while preserving a good sense of connectedness and directional+depth control, which is what hitting a ball is all about (mostly), right?…
      If you have good technique and strong arms, my guess is that you won’t lose many points by sending the ball long or wide, especially if your racquet is hefty enough (>300 g unstrung). After 2 or 3 weeks you may also notice some notching but nothing critical.

  4. Nice review Rui.

    I prefer the Dunlop string you reviewed before overall but Technifibre do make some good strings, I’d put Black Code in my top 5 polys.

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