They should fit, I'm not familiar with the difference between golf4 platform and golf5 platform in terms of braking, but 312mm carriers are the same accross the range I would have though?
Just to let you know, I got my 312mm carriers off my S3 to fit my B5 Passat, so my Passat is running 312mm brakes now, I just needed to buy brake disks and pads.