Personally, I think you should add one more mafia member and make it a gang of 5. It's generally best to keep the mafia around 25% of the players playing especially if we have a killer role now. As it stands, if the mafia doesn't lose a single member and eliminates the killer, it would take them 7 game days to achieve a tie with the remaining townies and win. That's a pretty tall order. Most likely they will lose a member within that time further setting back a win date and the longer a game goes on, the less and less chance the mafia has at winning. However, I am basing this math off the killer not making any hits but the killer is a wildcard and could also end up hitting mafia members so you never know which way the killer will influence the game.
Even with a 5 member mafia, it would take them 6 days to achieve victory without losing a member. However, with a killer and a possible bomber to take out other players, that date could move up or fall back. But even with a lose of a couple members before Day 6, the mafia could still be in good shape than a 4 man mafia losing 2 members before Day 6 and keep the game competative.
That's my thinking on the matter anyways.