Okay, I'm ready to throw out some guesses. My list of the four most suspicious players:
1. Thork (always suspicious, but still a good bet this time I think)
2. Marcus (Particle Person was right; a little too quick to agree with Thork in Round 2)
3. Skeleton (went against the mob in the 2nd round, even though PP wasn't a pod person, which is odd if he's an alien)
4. Cheesus (also a little too quick to agree with Thork, and generally [though vaguely] suspicious anyway)