It is an upgarde if they work better and dont seize... also they are much lighter than the cast Mk3's, Which I have also looked at... MK3 Caliper cost from GSF is only a few quid cheaper than MK4's...
They still seize (check out the MK4 section), they just seize
less. They also aren't much lighter than the MK3s - we're talking grams here. Also, given the hybrid of metals on the MK4s, they are more prone to corrosion where the two types meet.
But if you are not having bother with yours - it really isn't worth changing them.
But again, if you insist - I got my MK4 callipers from a scrappy at £15 per side, but ECP quoted £50 per side for MK3 units (with discount), but didn't have them in stock.