It really depends on your play-style. A low kill/time limit (preferably the lowest because of such a small amount of people), and an assault rifle (M4 or AK, any variant) unless you're into rushing or stand-offs. Good maps for tiny-teams are Egypt and Ship, as previously suggested. If you're into challenges and more exciting play, Ghost Mode or SnD are your best bet. Again, low settings are preferable because of the amount of people.
Happy hunting!