Subler is an macOS application created to mux and tag mp4 files.
The main features includes:

• Creation of TX3G and WebVTT subtitles tracks, compatible with all Apple's devices (iPhone, iPad, AppleTV, iPod, QuickTime).
• Muxing of video, audio, chapters, subtitles, and closed captions tracks from mov, mp4, and mkv files to mp4.
• Raw formats: H.264 Elementary streams (.h264, .264), AAC (.aac), AC3 (.ac3), Scenarist (.scc), VobSub (.idx).
• Full iTunes style Metadata editing.
• Metadata lookup for Movies and TV Shows on TheMovieDB, TVDB, and iTunes Store.
• Chapters lookup on TheChapterDB.