Having 5 players shouldn't be a problem for a casual game of Icehouse. It's mostly the real-time aspect that is the issue. things get pretty chaotic and hard to keep track of with more people not taking turns. Plus, if 5 people try to all reach across and place a piece on the far side of the table simultaneously, it's not going to work, someone will wait and may get hosed for being polite. That sucks if it costs you the world championship, but in a friendly game it's not such a big deal. Four players seems like the right balance of not too chaotic, but (compared to 3) more interesting possibilities for tactical alliances.