i wonder if there ever going to remodel the panzer...but the H model of the panzer IV's were a bit thinner skinned so they put sacraficial skirts to protect it from the 18 y/olds with bazookas. oh, and if you look at the wirble/osti the chassis is different in a few ways. the wirble looks shorter, but i cant be sure.
The Panzer IV, the Wirblewind, and the Ostwind all have the same chassis. The Panzer IV H has no less armor than the previous Panzer IV models, as a matter of fact the H has 30mm more frontal armor than previous models. The armored skirts were added to help with shaped charges (iirc), not to make up the difference in "weaker" armor.
Also, I'll have to go double check but the chassis under the Wirby and Osty is a "newer" version in AH compared to the Panzer IV. They do not have the same graphics. Maybe that means that when the Panzer IV is updated there will be less leg work because he chassis is already done???