Mechanically, armor is now balanced around proficiency (and the stat preqs of proficiency feats), skill check penalty, movement penalty, and the light/heavy classification, with the former adding your DEX or INT to AC and the latter not. Cost, aside from the cost of magic armor, is a minor consideration, and the cost of magic armor is the cost of a magic item of it's 'level.' +6 Full plate and +6 'cloth' cost the same (however many million gp), IIRC.
4e set out to make armor simpler and to make all armors worthwhile in some sense. They mostly succeeded.
The way it shakes out, though, Scale (which has no check penalty, and, with the specialization feat, no movement penalty, either); and plate, with the highest ACare genuinely good heavy armors, while Chainmail is just punishment for playing a 'leader.' 'Cloth' is prettymuch there for no one, since Leather has no preqs, so as soon as you find yourself with no feat choices better than a +2 AC (+3 paragon, +4 epic), you take it.. Hide is the best you can do in terms of AC for light armor, and pretty competative if your DEX or INT starts at 18 or 20 and goes up at every possible opportunity, but if you're not already proficient in it, requires a 13 STR & CON (and then a 15 CON to specialize), making it prettymuch an Archer-Ranger perk, as few wizards, warlocks or rogues have reason to boost /both/ STR & CON.