You can clear the cache on your device through the Go4 app by going into the general settings, scroll to the bottom and select clear cache.
You may be able to delete the cache footage on the device by itself if you navigate the gallery or camera roll on your device and look for the cahed files.
They may be in their own DJI folders and video might be separate from photos?
after afew videos it will overwrite the previous ( 2 GB ) files . they are low rez video files and not anything you want to keep on your phone to show . some of the photos are ok .