Calipers, yes.
Carriers, no...Mk4s have a different offset on the discs so sit 6mm further in.
But if you have a problem with a Mk3 caliper, you can use a Mk4 caliper on a Mk3 carrier and vice versa...you just can't use a Mk3 full set up with a Mk4 disc, or Mk4 full set up on a Mk3 disc.
Put another way, the disc and the carrier need to match. The calipers are the same regardless.