Lasers in series????
I think your tripped up before you start???
4 laser diodes electrically in series is a no-no.
Each diode has to monitor and control its power drive via its internal photo diode feedback. in series this wouldn't be possible, unless I've missed some obscure trick?
Unless of course your refering to physical series 'Line' of lasers? if so? then the following is still relevant.
Each of the 4 lasers should have there own control and drive circuit. This doesn't amount to a lot of circuitry, just same X4.
If say you used 4X 5mW laser diodes, with the 4X driver circuits, it will happily run on 3V batts, although at full drive for each of the lasers, this would equate to about 65mA x4 (260mA) and would run on 2X AA's.
I've knocked up a quick Schematic for the type of circuit you could use.
It will probably be harder to colminate all 4 laser to the one spot, physically speaking.
good luck
Steve