Can it be used for the Chinese light that runs on 4x18650? If so, brighter or lasts longer or both?
Yes, not a problem. Chinese lights that run on 4x18650 are what's called 2S2P - two sets in Series of two cells in Parallel. Total is around 8 volts (7.4 to 8.4v depending on whether they use nominal or peak voltage - same thing either way).
This 6 cell pack is 2S3P configuration - two sets of THREE cells in parallel. Same voltage. Longer run time because three cells gives you more mAH.
I use a 6 cell pack in the dead of winter when I need longer run time and the cold saps the cells power.