All four batteries run in the same direction. I would like to link 3 to 4 flashlights on the line.

What type of ac/dc adapter should I use ?

 

If these flash lights run on 3.7 volts then the choices are limited.

I looked for awhile and found one that could run around 10-12 flashlights for you.

http://www.alliedelec.com/mean-well-usa-rsp-200-4/70388262/

The price is right but it may take a week or more to get it.


Specs say it is adjustable in the range of 3.7 to 4.3 and is rated 40 amps at 4 volts. Not as small as a wall wart but it hits your target voltage and again one hell of a price from a good manf.

 

If you have long wire runs between lights then wire voltage drop might require more than 3.7 volts at the supply end.

 

Just use a common 5 volt power adapter with the appropriate size resistor for each flashlight.

Also consider that flashlights are designed for maximum brightness and not long service life so don't expect them to run for all that long before the LED chips start to degrade and dim considerably.