If you need exactly 5 for the rest of your circuit, you might consider a boost regulator like this
Use the onboard pot & tweak it to just where you want it.
The ATmega itself is not so fussy as the others have noted. I was testing a remote control for quite while with 3 AAs on VIN. Started at 4.5v, was still going well when got down to 4.25V.
Now have a 3.3V/8MHz Promini running on 3.7V Li ION battery on VIN and that is working well also.