Generally I agree, as accidental teamkilling, when you mistake an teammate for an enemy or when someone runs in your s-mine for example, should not be punished with -4 points.
On the other hand less penalty makes people go higher risk. So more people just shoot when unsure if friend or foe, throw that grenade in an unknown area with teammates nearby or shoot the HE shell somewhere where teammates might be for example.
So the inhibition level gets lowered and everyone just shoots, when it's a friendly it's just -2 points and it does not matter that much. So I think if penalty would be -2 only, team kills would overall slightly increase.
People might not all care about gaining maximum points possible on the scoreboard, but everyone feels bad when there are points removed you earned before, which usually makes you more cautious, so it does not happen too soon again. So the -4 is kinda imporant to keep the attention up and bring teamkilling to a minimum in my opinion.