I've played this with some friends at 4 player count and we found it too easy, have anyone experimented with adding more cards to the table at lower player number?
I wouldn't play with less than 5 evidence/weapon cards per player at 4. It just makes it too hard for the murderer. The game is easy to adjust in this manner to balance for your play group, so depending on who's winning more just add or remove cards as needed. It works great such that the game is still enjoyable even at low player counts.