Ran a bench test for the fun of it since I also wanted to do some conditioning on battery #4.
I formatted the card in the app (64GB Sandisk Extreme Ultra 80MB/s (Sandisk Pixtor).
Checked the card in the laptop to verify exFAT format and copy on firmware (double checking camera issue).
Restarted the P3P with the mSD card. Checked the settings (UHD 30fps (3840x2160))
Hit record, left it running about 1 hour, 20 minutes until DJI Pilot app crashed.
Result - 9 files all about 4GB, except the last at 3.2GB. Video's split at roughly 9:20.
Conclusion - Video's split at 4GB. This is a limitation of the camera firmware. The exFAT file system supports files up to 16EB.
Other tests:
Firmware upgrade: Nothing needed upgrading, including the new battery. The P3 may have copied over the full firmware log file since the initial upgrade to 1.2.6. I'm pretty sure there was no upgrade file, but will have to try again to reverify.