Can it be used for the Chinese light that runs on 4x18650? If so, brighter or lasts longer or both?
Yes, its the same voltage as a 4x18650, so will run just as bright for longer. The typical 4 cell packs are 2s2p (2 series 2 parallel), but you can get 2s, 2s3p, and 2s,4p packs that will have relative capacities of half, one and a half, double. Factor in the fact that often the cells that come with the cheaper lights are only half the rated capacities and you might end up with 4x the runtime from the battery pack below.
http://www.kaidomain.com/product/details.S024616