4AH? Holy cow - that's not nearly enough capacity to supply most eBikes for even a short distance. Most Li's should be 20AH to deliver enough start-up and full power current.

Nilar builds some very interesting NiMh choices - these aren't your usual NiMh batteries either. I've been sour on NiMh for years but these have got me revisiting the chemistry.

http://www.nilar.com/

I heard about some newer lead-acid designs that aren't your grand dad's lead batteries?

Lipoe is cool but you gotta be a good, decent, safe tech to mess around with that and my experience is 20% cell failure rate even under ideal conditions. Moral is - if you're gonna build your own LiP packs buy 20% extra cells and be prepared to troubleshoot battery problems.