Do you mean a plastic bodied 4AA torch?
It all depends on how hard you want to drive your LED. Plastics do not conduct heat very effectively, and the LED at full power will get hot if the torch is used for extended periods. This is unlikely to melt the torch but will shorten the LED's life.
I do make and modify plstic-bodied torches myself using modified PR Bulb bases, or replacing the assembly with a cluster of LED's, but this is using much lower powered 5mm LED technology.
Some of my metal-bodied torches are somewhat over-run, but the LED leg is heatsinked by the body of the torch.
I am looking to mod a large plsstic torch with luxeon technology, butb I hv got to find a way of increasing the torch's thermal mass before I go ahead with the mod.