Yeah.. I am undecided. My gut says "six", or at least five. because I like my cleric, but I also like my Druid.(*) In a four man team, there is little room for overlap (although two battlemasters tear through a horde of goblins like butter), and you"ll rarely see the interaction between the two.
With five, one can have the balanced four man party, and then add someone when the situation calls for it, without having to kick my best friends out.
(*)Eh.. Halsin will be on my team right? Come on, big buff custom model Elf.. The guy looks like Orion from Warhammer. No way you're just letting him provide counsel and health potions? Right?