Programming Attiny45 using Arduino Uno with weird behavior
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.