At 4 or 5 founders, it hinges on group dynamics, not just the individual personalities of the founders. You need excellent communication, clear commitment, and you absolutely need two characters in the mix:
- A "negotiator". The kind of personality that is always mending the cracks that inevitably appear in the group.
- A visionary. The kind of personality that is incredibly optimist and always has the destination and the path in view.
Smaller groups can forfeit these needs and function, and are thus easier to assemble.
I've never seen groups larger than five succeed, and I've seen quite a few fail.