Nikon 4300 uses EN-EL1 lithium ion battery pack. The 4100 uses AA cells. Li-ion packs are not nearly as easily available as AA cells. You can by AA alkalines in every supermarket or seven eleven. AA rechargeable cells are also easier to find. The EN-EL1 pack woud be available from specialized photo stores carrying Nikon cameras and even then, chances are it will not be a stock item. This is at least the case here in Europe. The 4300 can also be powered by a 2CR5 single use lithium pack. These should be available in most photo stores, but are expensive.
I suppose Nikon will supply the pack for the lifetime of the camera. If you go with a digicam that uses Li-ion battery, I'd recommend buying a spare battery together with the camera. It increases the cost by something like 40 Euro though.