I don't think there's a functional difference. I think 8's came in with the 1992 re-style, and I think only MoHo's got them. They're called "hand holes" for things like air service. The 4's will allow a little access by many adults. The 8's might allow an infant to reach in there, but then what's s/he going to do in there. Fleets would't accept them. Somebody figured 8's either looked better or MoHo buyers wouldn't miss the functionality of 4's.
I think Ford had a beef with the suppliers of 8's and simply dropped them, putting 4's on everything.
Either way, just install
Dually Valves. Much nicer with 4's and essential with 8's. They work with or without caps/covers/simulators.