Also, keep in mind that mAh is a rating of amps over time. So, if you have two 4v batteries, one rated at 600 mAh, and the other at 650 mAh, then they may both have the same instantaneous power, and thus may have equal light intensities, but the one rated at 650 mAh will last longer (in theory). As far as long-term longevity, the rating doesn't mean a lot, although the higher Ah rating may mean it has a better buffer for a slow decline (or it could mean they took shortcuts, and the battery won't last as long).
Power = Watts = Volts x Amps (and power over time = watt hours).
So, if you have say a 4V battery, and an 8V battery pack with the same mAh rating, then the 8V pack will be able to put out more watts hours, and thus more overall power (over time).
The nice thing about rating in volts and Ah (1000 mAh = 1 Ah), is that it makes it easier to calculate the battery characteristics when building multi battery packs in serial and parallel. Serial connections increase volts, and parallel connections increase amps and amp hours.