I've seen a few different ways discussed but this one seems like the best (I don't have the game to test it out).

• Each player starts with 2 Camps, +1 for each opponent. (So 3 players is 4 camps each, 4 players is 5 camps each)
• Players can damage any opponent.
• Events effect each opponent
• Players cannot Summon People or Punks in a column with a destroyed camp. (This is to balance out the additional camps)
• People and Punks already in the column after a camp is destroyed remain in their slot until destroyed.

Pulling from a different alternate ruleset I found online that was suggesting 2v2:

• First turn water allocation for 4 players goes 1, 2, 2, and 3. On turn 2 everyone gets 3. (3 players is 1, 2, 3, and then 3 for everyone)