I've just rebuilt my Discovery battery using eight 4250 mAH 'ultrafire' 18650 style tagged / protected lithium cells.
(Edit - 400 series style battery for reference).
Having seen all the warnings about lithium battery charging & equalising problems, I also found a charge controller board on ebay
that provides overcharge & overdischarge protection as well as cell equalisation.
The cells were supplied in pairs in shrink wrap, so I used each of these as a parallel pair on the basis the ones closest together out of the production line would probably be best matched.
I soldered each pair rigidly together with the tags angled in so they were just touching and used reasonably heavy link wires to chain the four pairs together.
They fit perfectly in the battery casing, I put both 2x2 sets in with the tags underneath to keep them away from each other. there is plenty of space over the cells for the (tiny) controller board.
The one I got is based on a Seiko S2854 controller and has power FETs rated at 8A continuous.
I refitted the original thermistor between the controller and cells.
The result is an 8.5AH battery which is, in theory at least, fully protected against overcharge, undevoltage and cell imbalance etc., at a total cost of (UK Pounds) £35
(You could just use four of these cells and still get 4AH+, or eight cheaper, lower capacity ones).
I've only used it for a couple of days so far, but it appears to be behaving OK.
If anyone is interested I can pull it apart & take some photos to show the details.
I can also post the exact batteries and controller if appropriate - I'm not sure about links to 'adverts' on here..