I'm looking to add a 3rd light to my current setup. I am using 2 Alienbee B800's. I want a 3rd light to use for a hairlight, or for the BG. Should I get another B800 or would I be better suited with a B400?
Well, the difference is that the B800 is twice the power of the B400. One stop power difference. It probably depends on how high a level do you operate your current lights? The B400 will operate doing the same job at 2x that B800 level. For example, if the B800 is currently at say 1/16 power, then the B400 will substitute there and do the same job in same use at 1/8 power, which is 2x higher setting.
If you want to operate either of your existing lights at a higher level, you could put the B400 there, and move the B800 to the third location. Frankly, I like to stay in the upper half of the power range, instead of lower half. Less red shift, and maybe slightly more stable.
The above seems a practical swap, because a background or hair light has potential to need a little more power. For example, the main light is normally very close. The background or hair light is more likely twice that distance, needing 4x more power for that reason alone. And if you might put gels on the background, or a grid on the hair, that often needs about 2x again.
With my ISO 200 Nikon, B800s seemed a little much power, so I bought B400s to use for main and fill, and I relegate my B800s to background and hair (just because I already had them).
The hair light modifier can also make a difference. I tend to mount a strip light above the subject - it is very forgiving but needs a bit more power that way. However, if I were to use a more direct and focused modifier like a snoot or grid, I would gain efficiency.