Basically, what the Lindens are telling us about open spaces sims, is that:
one normal sim runs on the core of a quad core server (e.g. Opteron), meaning 4 normal, full blown sims are running on a quad core normally.
An open spaces sim is 1/4 of a normal sim, so four of them are running on one core of a quad core server or 16 at whole, all of them put together serving the same prim count.
Being overused, the underlying maths about Open Spaces sims is that 1/4+1/4+1/4+1/4 is bigger than 1. Now that's at least what the Lindens are trying to tell us, which I for myself don't really believe at all.