Personally since I fly mainly digitals, 5.0 is bare minimum and that will be a very short flight. 4.8 and it's recharge time. Even small low to mid torque digitals will suck a lot of power out of most any batt.

Granted, the larger the number of MaH, the longer it will last, but down at 4.8 volts and mainly below you are really risking receiver dropout no matter what servo type you are using. The other item that no one has mentioned is the way the plane is flown. A sport flyer up doing pattern work at 4.8V may last a little longer than a 3D plane doing 3D aerobatics at 4.8V because when flying 3D all the servos are most likely doing something all at the same time, as in a hover so the drain will be faster. And that is not even worriing about digital versus non-digital

A quick once around the pattern may be fine at 4.8V but as I already mentioned, receiver dropout is a very real possibility.