Please realize that 4AA batteries only theoretically provide 6 Volts. They may do that if they are brand new. They will definitely never do that if they are rechargable.
Measure the voltage you assume to be 6 Volts. How many Volts do you truly measure?
The recommended Input voltage for a Arduino Uno is 7-12 Volts according to https://www.arduino.cc/en/Main/ArduinoBoardUno