Is it possible to directly convert .mp4 files to .srt format?
Converting MPEG-4 Videos directly to SubRip Subtitle Files (.mp4 to .srt conversion) is, unfortunately, not possible. You can imagine that trying to convert a multimedia file to a file type used for storing subtitles might be quite problematic.
The MP4 file type is, in fact, a container format, which might contain actual subtitle data. If that should be the case, use any available video editing utility capable of extracting the subtitles. We are, however, not aware of any existing conversion tool capable of converting MP4 to SRT directly.
In a word, direct mp4 to srt conversion is not possible.
So don't expect any so-called mp4 to srt converter or mp4 to srt free online conversion tool available.