If you're going through all the trouble of replacing batteries, replace the 3 alkaline coin cells with 2 lithium coin cells of the same size and a little more thickness. You'll get a bit more run time, a bit more brightness, and drop the weight by a few grams.

If you replace the resistor with one about twice the value you'll double the run time and half the brightness. I'd experiment with the resistance until you find something that gives you just enough light to read. That will max out the run time.

Oh yeah, those 3 alkaline batteries are about 40 cents apiece if you buy them in a 10-pack from an internet store. You save a whopping 5 cents by replacing them. If you have to buy the batteries from Radio Shack, you could buy all 4 lights for a lot less money. Therefore, if I was trying to be "green" I'd smash the the dead lights and throw the pieces in the recycle bin (so the metal could be separated out).