Ive got a similar kit on mine, the pipework is aluminium though, also from a golf 4.
I had to cut the end of the pipe down a bit where it sits in the bumper as it was too long, guessing theres more space in a mk4 golf bumper lol!
Nice sound, especially when you let the revs build up slowly and let your foot off a bit you get a long woosh LOL!
Fitment looks just the same, i moved the battery (just for fitment space, back now) removed wheel and pulled back liner, you need to fit the pipe work first then fit the filter on while its inside the bumper... Tight! Only thing i need is a small samco bend as one of mine is too harsh....