A zener diode seems like the cheapest and easiest solution, but is it reliable? Is there a benefit to using something like this http://www.sparkfun.com/products/107 ?
Just a diode in series drops things .7v. A little wasteful though.
Or you could simply replace your 4 x 1.5 volt AA alkaline batteries with rechargeable NiMh or Nicad units which run at 1.2 volts, giving you 4.8 volts which will be sufficient to operate your processor directly.