Sounds like a good group of players and interesting party of PCs. Are you planning to push them hard on their self-interest/amoralism?
On the NPC issue, I am with the chorus advising against trying to run a full 4th PC as GM. A companion character is a better way to go - and running it is more easily a table affair, as whoever has to look after it on a given turn won't have a big mental overhead doing so. For similar reasons I wouldn't recommend a warlock as an NPC - they tend towards the fiddly side, especially if you want to try and get striker-level damage out of them.
On the NPC issue, I am with the chorus advising against trying to run a full 4th PC as GM. A companion character is a better way to go - and running it is more easily a table affair, as whoever has to look after it on a given turn won't have a big mental overhead doing so. For similar reasons I wouldn't recommend a warlock as an NPC - they tend towards the fiddly side, especially if you want to try and get striker-level damage out of them.