Having a det at 4 makes it so that as an inno, there’s only two other people that could be the traitor, and makes it pretty easy to figure out who it traitor is. The detective doesn’t really add much in 4 player games, they’re just a confirmed inno. The main thing they achieve is giving the traitor someone to kill and get 2 points and buy something after just one kill, but that could be achieved by either giving them one point to start or making the first person they kill give them 2 points