4AAs meaning fresh 1.5V alkalines (and not sub 1.5V NiMH)? I measured the output of the 2 iriver AC adapters I have and they output 5.4V (not 5.0V as written on the AC adapter itself).
About WavPack, it seems like it is CPU intensive and will shorten battery life as a result. Seems like they committed the new on the fly recording/encoding patch with a new WAV codec called wav_enc.codec.
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FWIW, I have had no issues with the CVS build 20060717.
RE: WavPack, I like that codec a lot myself. But I'd still not use it... on the off chance that my iriver died while recording, seems like it would be easier to recover a WAV file with an improperly written header, rather than a WavPack (or other compressed) file.
RE: the AAs, either alkalines or NiMH. There's not a huge difference in 6V vs. the ~5.4V from the wall AC adaptor. And I did measure my NiMH fresh off being charged, at 5.96V, and no problems at all. I would imagine there's some slight "leeway" on the voltage amount accepted. Now, if you tried running 9V or more, you're probably dancing on ice.

I, too, use CVS build 20060717 with no issues (knock on wood).