Can it be used for the Chinese light that runs on 4x18650? If so, brighter or lasts longer or both?
No,
LED light circuits are designed to produce maximum brightness at the design voltage. They get dimmer if the voltage is dropped, but excess voltage not only doesn't help, it can overload and damage the circuit.
It's sort of like playing blackjack. 21 wins the hand, and getting close below that is OK but not as good. However 22 is bust.