I can't really think of anything off-hand that fits your description better than TF2 does. The maps may be a bit big, but I would think you guys could get around that with the different modes like CTF. Otherwise, Monday Night Combat wouldn't be a terrible choice, as the maps are generally smaller, but the characters are also slower, and it would be tough finding a balance where two people could defend the ball from the robots and still have time to kill the other team.
Honestly, I do think Borderlands would be right up your alley, even though it's pretty much co-op only. It's definitely the best online experience if you have only four people. Honestly, it's probably my favorite online experience of the past 5 years.
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Xbox GT/PSN name: TatteredUniform
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