Why can't I convert a video file to an iPhone version through iTunes?

I keep getting the "An unknown error occurred (-4)" message and can't convert a.mp4 video file I got from a GoPro so I can put it on my phone.

I tried downloading it from Dropbox, (computer to mobile app), but I get prompted that the file is not compatible with my device.

I've spent a great amount of time on Google and nothing is working. I was hoping someone could help me out here. Thank you

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