Just photo copy a the player mats and buy some extra cubes of a different colour and presto you have a fifth player setup.
I did this to add a sixth player to one game. Photocopy mats for choice of bacteria or virus, used Risk pieces for markers and we were off to the races... er... doctor's offices... or something like that.
In case anyone else was wondering the same thing: I contacted the designer and apparently the 5th player expansion is also a KS exclusive. It may possibly be available with another future expansion.
Very disappointing. Not sure I like the game enough to keep if I can't upgrade it to 5 player.