handbrake cables are not pre/post 90, they're pre/post 87. You can check what calipers you have by looking at them, handbrake cable to top = pre-87, handbrake cable to bottom = post 87.
I've heard you can fit post 87 calipers to pre 87 car by fitting them the wrong way round, but this results in the bleed nipple being at the bottom so when bleeding you need to whip them off the car and hang the right way up.
Oh and you could get a 256x22 rear disc setup on MK4s models, can be fitted but takes a bit of effort as you cant get MK2 style rear discs in that size, so you need to convert to MK4-type rear stub axles, but MK4 bits wont fit so you need late Polo/Ibiza bits plus then find a disc that fits. Theres a thread on cgti about converting to MK4 style stub axles if you really want to do this, but iirc I dont recal any easy rear disc fitment.
