Help! Recorded for 8 hours but only 4 hours are saved

No. You severely overloaded your iGPU and its encoder. It was not able to encode almost all frames. There's nothing in your video file.
 
The video encoder was severely overloaded, so it was able to only output something that will probably look like a slideshow and not a fluid video. However, at some point in time, it choked completely and stopped output.

 
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