We did a good amount of testing trying to make a fifth player work and, at least for the base game, it just doesn't. Part of the issue is total number of actions over the course of the game... part of the issue is number of available spaces. The board/game is balanced around have six action spaces to choose from and with five players that means all but one is used. To make it work we'd have to add a whole other area of the board. It's an area for a possible future expansion, but not something we can likely handle in this campaign.