Howdy 1-3-2-4 and welcome to CPF,
It's possible that changing the four 1.2 volt batteries out for 4 Energizer e2 lithium batteries might make it a bit brighter...... but might also cost more than the light is worth.
Depending on the light, it might also be possible to replace the current LED with a newer, more efficient LED. You might want to spend a bit of time over in the Homemade and Modified Lights forum:
http://candlepowerforums.com/vb/forumdisplay.php?f=17
However, I've got a feeling you might be better served by doing a bit of holiday shopping for a new light. There are many inexpensive lights with new generation LEDs hitting the shelves now, so spend some time reading about them here to help you find one that will fit your needs.
By the way, in case you missed it, be sure and check out the CPF
Welcome Mat page...... it's full of links to threads and articles that will help answer many of your questions:
http://candlepowerforums.com/vb/showthread.php?t=172991