You can make the Mk4 locks fit a Mk3 by filing the holes out. The doors are easy, but the Mk4 boot lock is loads bigger and you have to remove quite a lot of metal.

If you want to fit the petrol cap you have to change the fuel filler neck, and you can make the ignition barrel fit by filing the hole out but you will loose the steering wheel lock.

I've done all of the above, but tbh I think the Mk4 locks are no better than the Mk3 ones so I wouldn't bother doing it again.