I don't really care, but giving it 4 shots would be overpowered as fuck, simply because that'd mean if you're halfway competent you get 5 free kills as traitor.
good point actually
If there are 4 traitors in an 18-player game, that leaves 14 innocents to be killed. If they all get 4 barrett kills plus a knife, they can easily take out the entire server with nothing but basic tactical skill.
2 shots, means 3 instant kills counting the knife which translates to 12 total on a full server.
I say one shot.
However, letting the traitors buy a normal barrett with normal ammo pickup and normal damage wouldn't be so bad.
After a little (really, not that much) consideration, I think the only surefire way to know what's balanced and what's not is to test it.