First question that comes to mind is: why bother running external power on an on M1/D100?
Anyway, yes, you could run 3 AA alkalines in a 4-AA sled to get 4.5v, but you'd need a dummy cell for the 4th. Better option, IMO, is to run 4 rechargables for 4.8v. But I think the Sony portable decks are BEST powered internally by rechargables - running 4xAA rechargable externals won't buy you much run-time.
I don't know for certain if the M1/D100 will rollover to internal batts after externals die, but I think not.
FWIW, I get 4-5 hours on my 2000 mAH rechargables. Since the M1/D100 only take 2 AA, you can get a set of 4 and be set to run a looooooong time. I actually have *16* AA rechargables that I use for a variety of devices, so I'm ready to run a looooong time if need be.