lyrical10:
I am using four rechargeable AA batteries in series to power an arduino nano. Should I be using the 5V pin or the VIN pin? Also would this be a regulated or unregulated power source?
If the type of rechargeable AA batteries (4 in series) is less then 5.5 vdc when fully charged then wiring them to the 5V pin is the best method. Applying +5 vdc to the Vin pin won't work as that is below the on-board +5vdc regulator's input voltage requirement. Technically a battery is not a regulated voltage source as the battery voltage will decrease as it discharges. However the arduino board will function fine with the battey, with the only disadvantage that analogRead values will lose some accuracy as the battery voltage decreases.
Lefty