Beyond AA NiMH - Best Li-Ion pack config

[burpee]

What would be the next two or three more powerful battery configurations to power a flashlight with an attention to runtime.

I bought the Fenix LD 20 R4 and Quark AA2 R5 that both produce the desired light power (205 lumen) but have poor run time. I measure well under 2 hours.

So what are next powerful formats - 2x18650? What battery source will power the type of LED that produces 200 "real" lumen and will run at max for four hours? Does a 2x26650 flashlight exist?

Thanks.