https://www.rcgroups.com/forums/showthread.php?2458896‑DJI‑Phantom‑3‑camera‑wiring‑diagram 
• https://phantompilots.com/threads/phantom‑3‑advanced‑camera‑hack.43126/ 
P3P uses Sony EXMOR sensor, while P3A contains a cheaper equivalent by Panasonic.
For the camera/gimbal board, the Pro model seem to have separate video compressor SoC ‑ the DM365 chip. Noticed it here:
 New P3P new cooling fan on 4K camera
The DM365 is not powerful enough for 4k, but it can compress the live video feed (FPV). This takes some load out of Ambarella SoC, and allows it to focus on compressing stream for the SD card. This is why existence of this chip makes 4k possible.
Enabling 4k in firmware would make the Ambarella overloaded, so there would be frame drops on both recorded video and live feed. But still, possible to enable.