By 6-volt dry cell, I assume you have the square camping lantern style battery?
The only 4.5V I’m able to find is listed as a 3R12?
Since I don’t know anything about your flashlight, it would be difficult to say, but an inexpensive driver circuit like this one:
http://www.dx.com/p/18v-5w-cree-circuit-board-for-flashlights-16-8mm-5-5mm-26110
…would step down the higher voltage and let you power a single emitter at about 1.25A. If your LED is a true Cree 10W, then I would expect it to be an XM-L T6 or similar. This driver would not give you full power, but would still be enough to get lots of useable light.