OK... Ran the R-4 off a 9.6v 3000mAh RC battery till it ran out. Recording 4 chan at 24/96 providing phantom to each chan I got right at 5:30 recording time.
OK... Second test... Ran the R-4 off a 9v 5400 Lenmar DVD battery that mmMatt sent with the R-4. I got about 8:40 before the R-4 shut down. Have seen these new for around $55. Did not look hard you may be able to find them cheeper. One thing I really like about it is the LEDs on the front to indicate how much charge is left.
Over all, when I got my UA-5, I was able to get 4 of the RC batteries + charger for the same price as the Lenmar. If mmMatt did not send the Lenmar I would be perfectly happy with the RCs. However, for my everyday taping I will probably use the Lenmar (the LEDs are really nice.) For festivals or other times when I may not be able to charge I will probably just bring everything with me.
Bottom line... for a typical show both a single 3000 mAh RC or a Lenmar type dvd battery should be more than enough power.