By default, GPS Pin 4 is a general purpose input pin. For advanced integrations, this pin can be changed from a digital input to a digital output in the Advanced Configuration page.
The setting is called Top of Frame Output and is for advanced integrations only, when external hardware, such as an RTK GPS receiver, needs to know the precise moment the RedEdge captures images. It is available in firmware version 1.28 and higher.
⚠ WARNING: Enabling Top of Frame output changes the PPS pin purpose from a digital input to a digital output. Damage to the camera may occur if another output pin, such as the PPS output on a GPS receiver, is connected when Top of Frame Output mode is enabled.
If you use this method, you will have to update the metadata of the images with the RTK GPS information in post processing. As a backup, it is a good idea to send GPS information to the camera during flight using the Serial or HTTP APIs.