IMO classes should only get forth specs if, like druids, they already have a double role spec, or if there is a concensus that the pure dps classes should become hybrids.
For rogues specifically they don't need a forth spec right now, they need a warlock style overhaul to make the three specs feel different. When I play my rogue the only thing that really feels different for me is whether I put a dagger or slower 1h in my main hand, and which dps cooldowns I have available.