MP4 ?Again: without knowing the first thing about the clip, there is no way of knowing. It may or may not work just fine. In which case it's irrelevant what I think.
In my question I said "extract the subtitles contained in a video".Still says nothing about whether it has embedded subtitles or not. So you tell me. Is "TunesKit" able to extract any subtitles?? If so, then I'd say any further discussion is extraneous. You have what you want.