I have used 4 trimmer lines with a thickness of 4 mm. Evaluation in a difficult environment [grass over 1 to -1.70 meters and brambles [not many] -1]-The most durable is the vortex, so first-2]-Second is the nakayama 3]-The hunday 4]- kraft [not in terms of durability but because although it says it is 4 mm, in reality it is not. In detail, if all 4 had the same price, then the vortex would be the top choice. However, it is expensive and has fewer meters than the others. The nakayama, if it hadn't become more expensive this year [last year I bought it for 8 euros without shipping while this year it reached 12 euros without shipping] would be a better choice than the vortex. The hunday, buying 2 for the price of one compared to the other two [vortex-nakayama] is a fair purchase but with a basic condition. Use in a less difficult environment. What does this mean practically? First, I use one of the first two, preferably the vortex, and then for economy reasons and since the grass that will regrow will be smaller, I use the hunday. However, if you are a professional or want to clean in a difficult environment, then prefer the vortex [twisted 4 mm] and as a second choice the nakayama if you find it at a much cheaper price. The hunday twisted 4 mm is not of the corresponding durability, it breaks and wears out very quickly at a ratio of about 1 to 3 compared to the other two and between vortex and nakayama, the vortex is more durable at a ratio of 1 to 1.5... I hope I helped... I do not evaluate the kraft because it is not 4 mm although it says it is, so I consider it not durable in the 4 mm category... you may need to buy slightly larger millimeters to be able to evaluate it...