There are 4 arenas but all are the same "server" -- all have the same perks, the same tracking, the same setups. I'm guessing it's a coading trick to split up 1 server into 4, and that they all use the same coad, so that the skins can't be changed.
That's my wild uneducated guess.
I don't think I'd like it. Sure there are P51 skins that can't be accepted because the p51 slots are full, but what if 1 skin is available in the late war arena? It's dead in there. Nobody's going to see it. I think it's better to have the same skins in all, and just rotate the p51 skins more often.