The MK4 mech isn't any better than the late MK3 units, and the MK4 calipers also have problems with the nipples siezing in + they corrode quite badly due to them being unprotected aluminium. If you do get a set make sure you paint em, plus you have the added expense of the MK4 adaptor cables! The ONLKY advantage MK4 calipers have is the weight, I'd go with MK3 units every time unless you can get MK4 calipers really cheap!
MK4 pads should fit I think, but its not the caliper that determines what pads you get, its the carriers. The pad slides into groves in the carriers, the caliper just fits over the top of the pads.
Edit: Its the cables that freeze usually, if the boots are ripped water gets in and the inner cable freezes in place
