My most recent experience in this area is with an OS 5 Cyl 4 Str radial engine. I was having problems keeping all five lit. After much experimenting I determined that OS F plugs pulled just under 3 Amp each from a Nicad. I tried a number of combinations of batteries and micro-swithces, but it got very messy.
I eventually settled on a small onboard unit and battery from 42-percent-products. I had some doubt that such a small inexpensive unit and battery would be adequate. The battery pack consisted of 3 c size 4800 mah wired in parallel.
I first wired a bench mock up and was surprised to find that all 5 plugs glowed quite brightly for one hour. So I expect to get a good days flying from one charge
The current pulled by a plug will depend on the applied voltge. Assuming you intend to use Nicad and have very little resistance from wire and other stuff, then allow for 3 amp per plug.
If you want to go the easy route and run all four engines from one unit and battery, contact the 42-percent-products