Tecnifibre Black Code 4S is a co-polyester tennis string with a square shape and four spin-friendly edges. For our playtest team this string's most undeniable characteristic was control. Put simply, Black Code 4S enabled us to execute our most powerful strokes without fear of overhitting. Our ability to take massive upward cuts at the ball -- especially when combined with this string's four grippy edges -- was a recipe for massive spin. Another prominent feature of this very spin-friendly co-poly was its firm feel. Although we found Black Code 4S softer than the original Black Code, it did not forgive mis-hits as well as Babolat Xcel, Tecnifibre NRG2 or a soft co-poly like Polyfibre TCS. If our team has it right, this string is not for the uninitiated. It is for the poly veteran who wants a well-crafted tool for converting big swings into spin-loaded missiles that drop sharply and accurately.