unless you have car that has adjustment on the rear (which you dont) they dont adjust the rears at all.
However the reason you get a 4 wheel alignment over a 2 wheel is:
If you do a two wheel alignment then the two front wheels will align with each other, but thats not to say they are aligned with the rears, you could even find the car crabs in extreme circumstance. If they bodge it you may even find the steering wheel is at an angle as the front align with each other only, and not the rears.
So if for example someone was doing a two wheel alignment, they may simply adjust one side to bring it in alignment with the other side.
However most places if they do it properly should adjust both so the steering is correct.
With 4 wheel alignment, they take the rears as the reference point (and there is some assumption that the rears have not been knocked out of konk).
Then using the rears as the reference point they adjust the fronts to be aligned with each other and the rears.
They should before starting adjustments and measuring clamp the steering wheel so that when all the work is completed the steering wheel is straight.