Help!!!

I have got mk3 gti rear stub axles
Can I mate them with 4 stud discs as I believe originally they would be five stud on the gti models?

yes you can using mk2 discs

Will any 4 stud mk3 discs work ?
As when I tried fitting today the disc didn't feel seated on the stub axle even when the bearing was in place

mk3 4 stud rear discs are the same as mk2 gti discs so yeah they would fit

if mk2 gti discs dont fit your stub axles then the wheel bearings may not be fitted properly, or you have an old bearign race stuck to the stub axle. pics?