Mold stacks at BBS woodshop take four boards at a time, so they are marked as such, I being the top, IV being the bottom.
Normally I would go with the simple I being the steepest or at least having the most concave and IV having the least concave, but a few I have had recently don't quite add up to that, even though for the most part, it is almost always true.
Guessing it could also have something to do with the presses, eg if some presses were more mellow than others, the full stack of four decks would be more mellow overall than other stacks, which could also mean the I of a mellow stack is flatter than a IV from a normal stack.
Without trying to get too crazy with it all, there are differences, but none so much that it would mean one board is completely different from another, as they both still have the same shape, same concave, same kicks, with some being overall a couple of degrees steeper than others.