Neddy,
You should not let your pack drop below 1 volt per battery.
If this is the most common 4 cell pack, you've got one more volt to go.
Additionally, each cell is rated at 1.25 volts or 5.0 volts for a 4 cell pack, 6.25 for a five cell.
Why quit when you just got to the rated voltage, plenty of room left.
Good Luck,
KW_Counter