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fq has mp4 support https://github.com/wader/fq is quite visual but is a CLI tool (for now). It has a REPL and query language to poke around. Disclaimer: i'm the author.

Other mp4 tools i use are https://gpac.github.io/mp4box.js/test/filereader.html and the tools from bento4, also ffmpeg -v trace can give some useful output.

You could also try the Bitmovin MP4 Inspector chrome extension: https://github.com/bitmovin/MP4Inspector

It might not have the prettiest UI, but I've used this one several times when troubleshooting encoder/muxer issues: https://github.com/essential61/mp4analyser.