are you going to go with a scale 4 blade prop for it?
The easy answer is to go with 1 of the "glow replacement" style numbered motors, an e-flite power 10 or O.S. OSMG9510
Both of these are designed to replace a .10 sized glow directly, and will likely have more than enough power given there are 4 of them pulling the plane. The tough part you may have is fitting the blades... You are limited to about 8" props if you go scale...
The real question is, how much is it going to weigh when done. What are your estimates.... if you are in the 6-8lb range the .10 sized glow replacements will fly it nicely, you might even be ok up to 10lbs.
My suggestion, or, if you go to the electric B-17? thread, how I decided on my motors
I decided on prop size I had to live with due to the planes size... for you thats either 8 inches or 8.5 inches.. so 8 inches... you don't really gain anything by going to 9.3" spacing as props typically aren't sold in 1/2" sizes.
I then picked a motor I thought would work.. in my case an e-flite power 15.. i measured the area I had to work with in terms of fitting batteries and made some trade offs in terms of weight vs capacity... decided on about a 2200mah 3s pack, for each motor... should give 8-10 minute flights, and at 176 grams a piece helped minimize the added weight of the batteries
I then bought 1 of each, and an ESC that was way oversized ( 80 amp ) and a wattmeter. Hooked everything up and figured out my current draw which told me how big an ESC I really need. It also gave me some confidence that the system would be sufficient for what I wanted to do.
From there I sized my final ESC's and bought the whole package...
there is no one right answer here unfortunately... and not a ton of people flying multi-engine setups... trial and error is your friend here... I would encourage trying a couple things and seeing what works and go from there....