If you take a look at Attiny45 datasheet, you will find at the very first page:
Operating Voltage
1.8 - 5.5V for ATtiny25V/45V/85V
2.7 - 5.5V for ATtiny25/45/85
Your circuit will have unpredictable behavior out of this range. If you want to use four AA cels, you can organize them to have a 3V output or use a step-down voltage regulator.
rvbarreto
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