The mk2 rear callipers have external springs and tend to stick where as the mk4 callipers have internal springs and are much better as the springs don't get corroded, mk4 callipers also use the same pad carrier and handbrake cable as the mk2 so are so east to fit, literally 1 bolts and a brake pipe, however they have a banjo fitting on the brake pipe so you need different hoses. Also mk3 and mk4 rear callipers are the same exept mk4 ones are aluminium rather than steel.