3.6v regulation

[jmgelba]

3 batteries at 1.2v will only give you 3.6v for a short while depending upon how much current your circuit requires. You will hit 3.3v fairly soon.

Can you use 4 batteries?
4 x 1.5 = 6V Plenty of headroom for an LDO
4 x 1.2 = 4.8V Still plenty of room for an LDO

Is this high volume, whats your cost for the circuit? Is there enough budget to use a switcher?

Here is a simple circuit using Linear Technologies SwitcherCAD program.
3.2 to 6v input, 4.0v output at up to 350mA.