You could make a constant current LM317 driver. Its very cheap. Only downside I can see is that 4 AAAs wont cut it. A red diode needs around 2v and the LM317 will drop around 2.5v. 4 rechargeable AAAs is only 4.8v. You would need six cells for this to work.
You could also look for a buck driver. Just use the search function on the bottom of the page.