I do agree that two separate maps would be required, but the work involved in porting over the Mk. III skins to the IV should be minimal at best IMO and I think it'd be pretty harsh to expect existing owners to pay full price again, particularly for the £10 skins.
Oh, I absolutely agree, just saying I'm not surprised they're still charging separately for them. Despite the similarities they will treat the Mk.IV as a separate ship and sell skins separately. It is what it is. I'll test out he Mk.IV and probably only invest in a skin if there's a chance I'll use it for more than novelty's sake. I do love my orange rattler currently parked in Alioth