I haven't converted a four motor, but have done a few twins on electric. You will need a seperate ESC for each motor if you are going brushless. You can use one battery if it can handle the amp draw rate. At around 15 lbs. you will need about 1800 to 2000 watts to fly the plane well. If you keep the motors sized right you could probably go with one battery maybe a 4S or 5S pack. You should be able to use something like an E-Flight Power 15 or 25 for each motor and get great power. You will need to put an amp meter on the plane and see what it's pulling at full power. One battery would make it easier as you would have less wiring issues. It would be good to counter rotate the two motors on each wing in case you loose one side for some reason, you may still be able to land the plane. So far, I haven't ever lost an electric engine on my twins. I have lost both of them becuase I flew too long, but that's no problem if you have enough alt. you can still land.

Keep us update on your progress. I would love to convert a B17, B29 or B24. I have done a B25, P38, OV10 and A10 so far.

Should be a fun project. Good luck!