How do you get all four player to "fight"?

Monsters spawn at a preset rate ... something like 2 monsters every 4 hours ... that's why when you wake up in the morning (presuming you don't get up at 2 a.m. to play tapped out) more monsters than usual are there for you to attack.

I don't know what the algorithm is but I assume there is a similar rule that determines how often you get the higher power monsters where you use 2 fighters at the same time.

Check out the strategy threads ... I'm pretty sure this stuff is spelled out fairly clearly.