You cannot exactly convert subtitles (such as SubRip .srt files) to a MPEG-4 video (.mp4) directly, which mean srt to mp4 conversion in this sense is not possible.
The only thing feasible would be to import subtitles to a video file. This can be done with lots of various video editing tools or even dedicated subtitle importers.
What is also doable is to generate "dubbing" audio tracks based on video subtitles that can be added to back to the multimedia file as alternative audio track. This is often done by users that downloaded movie/anime in foreign language and want to add track in English or their native tongue.