TTL will not work to meter a hair light (or a background light). Because TTL tries to make the center subject bright, which is not where the hair light is (nor the background light, which just sees a black spot in the center, blocked by the subject). Those will have to be a manual mode flash.
If wanting to use the Commander, it could be a SU-800 commander, or a SB-800 or SB-900 used as Commander on hot shoe. Those will individually control up to three groups (A, B, C), and the Main and Fill lights could still be TTL. Flash on the hot shoe could be the fill light (a fill light NEEDS TO BE near the lens axis anyway), or it could be disabled and only Commander used. But four lights will still be a problem.
Some optical slave sensors (digital slaves... S2 modes) can work to ignore one TTL lights preflash, but it cannot work with the Commander. The Commander situation is that there are SEVERAL flashes before the final flash, and a simple optical slave simply will be fooled, and will trigger far too early (ineffective, not useful). Optical slaves need a manual flash situation.
But frankly, TTL offers very little for studio portraits anyway, less control, and they vary, more detriment than advantage. If using multiple manual lights, you simply set up the manual lights like you want them (full control, but a hand held flash meter becomes indispensable), and then the lights do not change during the session anyway. You don't want your lights to be changing during the session.
So if you use your three lights all in manual flash mode (triggered any non-commander way - can add a inexpensive slave or PC connector to the SB-600 foot), you are good to go now. It will become very simple, and you will have no problem. But again, a hand held flash meter becomes indispensable, to set each light to a known level, and specifically, to be able to repeat the same lighting in the next session.
