I can also confirm that my M4P, shipped from Japan, also has RID. Interestingly, It didn't activate when I turned on the drone, but as soon as I lifted off, it popped up on my phone. As reported above, it is the serial number of the drone.
Straight from DJI:
Hi there, the Mavic 4 Pro has a RID. RID is the identification and location information provided by the drone in flight that can be received by third parties. In order to better monitor and manage drones, the US government...