Hardened cop/detective
Writer/Artist/Musician
Yeah, I've been watching Castle recently, but I still think that the two would make for interesting teammates.
An actual member of a military force.
I think that the role Bill had in the first game explained a lot about their survival. Two weeks after first infection they somehow hook up, and Bill is the one who teaches them about long term survival in an unsafe environment instead of just short term running from place to place. I'm not sure any of the characters from he second game really had enough explained to make me believe they would be equipped for long term survival and hiding out, or even handling all their guns as expertly as they do. All the characters from the first game had their skill explained in various ways, and even without it Bill could have taught them all.
And a fourth. I think this should be someone more innocent, like Zoey, to promote the idea of a band coming together with one goal in mind, the protection of an innocent. A hardened cop, a writer and a military or recently ex-military trio find a high school senior or junior in college, smaller, weaker, an much more frghtened, very little training, and then they traing, probably a female, her up.
Valve are excellent at telling stories without dialogue or exposition, and despite Zoey's badass nature I got the feeling that the story told there was about the others trying to protect her, same with Ellis in the second game, both younger and less experience, so a natural protection instinct kicks in. Great storytelling.
So that' my four:
Cop
Raconteur
Soldier
Innocent.