...need to know if the system can handle up to a 4S battery pack?...
FT changes components occasionally, the ESC you have may not be the same one sold today. There should be a label on your ESC which will indicate if it can handle 4S.
I run a 12x4 or 11x4 on my FT3D, my motor is a 3536 on 3S, this setup gives great performance. A 12 inch prop might be a bit much for your motor on 4S. I would start with a 10x4 and see how it goes.
I do a bench test with any new prop setup. Run the motor at full throttle for 10 seconds. Then stop and check the temperature of the motor, esc and battery by touching them. Warm is OK, but if anything is so hot you don’t want to hold on to it, STOP, that’s too hot. You need a smaller prop. If it passes, try a 30 second run, then check the temperatures. If it passes, try a 60 second run. If it passes, you can go for a fly. It’s a good practice to periodically check the temperatures after a flight. What works when it cool outdoors, 70, may overheat when it gets hot, 95.