Constant voltage converter?

[RCatR]

I'd like to find a constant voltage converter that I can put in series with a flupic; allowing it to be used on 2 rechargeables.

Something that isn't a resistor and will drop 9V down to 4V so I can stick to 123's and r123's for powering a modified E2D

Any suggestions? Prehaps a way to convert a downboy to constant voltage?