Weren't you having problems with the three you have now? I'd say three is max for a 180, I'd wait until the upgrade. Once that happens, you can introduce the 4 or 5 at the same time, or best order suiting for your needs (smallest, least aggressive forth). Having plenty of liverock with multiple caves and hiding places will help. Each fish needs to defend its own territory. Also with emperors, they tend to need to be the boss of the tank, make sure he doesn't get picked on. Maybe try the eggcrate trick when you introduce them.