The best way to use Rechargeables is to keep them in the same set, whether using two or four at a time. Mark them 1,2,3,4 or a,b,c,d and keep them together with an elastic band as soon as you remove them from the equipment and until next use.
As has been mentioned above, it isn't a good idea to mix these batteries and have them at different charge levels. The stronger one(s) will try to charge the weaker ones and can easily get very hot or even catch fire. Always charge the set together. This method also extends the battery's useful life.
If you have to replace a failed battery of a set, either dispose of them all, split a set of four into two, or if keeping a set of four charge the new battery on it's own first, then charge it again with the set of four. Most modern chargers carry out a 'discharge' prior to charging and this will balance all four batteries. From this you will understand why it's not a good idea to leave batteries in the equipment for long periods of unuse. Always remove them. Better safe than sorry.
I find our local weekly market the cheapest place for rechargeables and buy them as a set of four. The make I have been using for a long time now is the Uniross 1800 maH. The 'maH is important (whichever brand name) as these are the strongest for using in digital equipment such as cameras, portable radio/tape and DVD players. They always last a reasonable time, even when using flash.
I trust that this is of some interest.
Cheers.
BillEWS.