Despite 4e's apparent attempt to make the cleric/leader somewhat less vital, it failed utterly in allowing a viable party without one (IME). If you plan on challenging your players at all, you'll probably want to fudge potions somewhat (maybe add some steps in the power progression), make them cheaper or hand them out way more often than suggested, and strongly suggest that they take Quick Draw ASAP. A leaderless party, IME, is just flat out dead meat before very long at all when using the 4e system without any modifications.