1. G4 calipers are alloy so probably best not powdercoat 'em
2. They have an internal handbrake mech, thus they don't seize - better than G2 calipers
3. They are alloy, so lighter unsprung mass - good
4. You will need a set of 'conversion hoses' with banjos on them as the fittings are different.
5. It's late and I've realised I've not really answered your question as it's about the carriers.

6. Use the Golf2 ones - they fit - it's the calipers you need in this situation.