Maybe a little late...it was me who played with 4 NiMh cells. It does work but Brian's memory is correct - when the 4 batteries are fully charged the voltage is just a little too much for the JB3 and it won't power up. I now just run a halogen bulb for about 30 seconds on the battery pack and it drains enough to work after that.
I've done the 4 x NiMh (using 4/3 A size cells - 3000mAh), a 9.6 NiMh pack regulated to 5V, 9.6 NiMh pack with a car adapter at 4.5V and 12V SLA regulated to 5V. They all work. But really, I'd just go with a 2nd Creative battery if I had to do it again.