Video stopped recording after ~31 minutes
This issue of video stopping at the size ~3.80GB seems to be file size restriction embedded in the camera app, irrespective of the filesystem your SD card has, or the space left in it. (Note that some camera apps also has a default limitation of 1-hour video duration, however, it is not related to the relevant issue here.)

My default camera app in OnePlus One device  gives me a toast "File size limit reached" when this file limit is reached by the camera recording 4k video for duration 8:20. Depending upon the resolution of your video the duration will vary. 

It is also to be noted that the industry wide standard of 1GB is 1000MB compared to 1024MB of computer science, and so, (4000x1000x1000) / (1024x1024) = ~3815MB or ~3.8GB &rarr; where the video seems to stop finally.