Yeah, -4 does seem steep especially when it isnt a deliberate teamkill. But its a tough balance, cause DICE and EA have to have some sort of disincentive for killing teammates. Problem is the people that really want to tk and ruin other people's fun will do so without impedence, they just don't care. So the -4 is really supposed to be a disincentive for people that think about killing a teammate for a vehicle, they have every intent of winning the game, just not outside of "their" vehicle. -2 is too low, if somebody is going for their explosive badge, if the score were only -2 for a tk, they would gladly kill one teammate for one enemy assuming the server doesnt kick after so many tks, cause they would break even, +2 for dead enemy -2 for dead ally.