1.1v per cell, or 4.4v for a 4 cell pack is about the lowest you should go, though I've heard of going to 1.0v, or 4.0v. Certainly, at 4.8 you are still on the "flat" part of the discharge curve, but at 4.6 and 4.4 are you are at or over the cliff.

Now, you can actually go a fair bit lower with out damaging the cells, as long as none of them reach 0 before the others.

But you've lost useable power for your system around the 4.6-4.4 point, and many guys recharge at 4.8, which is fine. I'll fly a short flight on 4.8, if I know the pack and the model. At 4.7 or lower I recharge for sure.