I'm sure others may have better suggestions, but I've been playing with this kind of thing myself. I would suggest either using the 4 nimh, or getting rid of one of the alkalines and run it regulatorless. I have been pretty surprised here. I have a 3.3v uC in a remote project here in sleep mode that I check every once in a while. It's running off a single 3v CR2032 coin cell battery, and it's been running for about a month.
Obviously this depends on the other power demands of your project, but if mine can run on a single coin cell for a month, you should be able to run with 4AA's for much much longer.
There are a few design rules to consider for reducing sleep mode power to the minimum. I think someone else listed them in a much earlier thread.