Yes, the 4.5 for Atmega8 @8Mhz I use are good, I mean slightly other thing: you mentioned that voltages of both devices should not be different more than 0.5V.
With Arduino board (that I use as USB-UART bridge) at 5V and Atmega8 with 4.5V (or even less if batteries are old) the difference should be greater.
Is this ok?