Yes, certain SUV rentals are 4WD (I've been driving them continously since 2005, courtesy of National's generosity - and a great corporate rate). And yes, 4WD does help in inclement weather conditions.

BTW, my advice to the OP - escalate this matter as far as you can. Clearly that particular National location violated their own terms - and you shouldn't have to pay that ridiculously arbitrary premium.