You will get a bit more exp with 5 than 4 cause of the group bonus (I believe) so I would just go ahead and get the 5th. Pally priest and 3 mages is quite the AOE powerhouse. Even at lvl 21 my team just spams that That gives you more zhevras, and more gifted levels too, so you can get a free 60 or two once you are done.