pramod@: this was discussed in the past and we decided not to support MP4,
for a number of reasons, that boil down to it being an unclear use case,
i.e. to support MP4 we would need to add a considerable amount of source 
code, and correspondingly increase the binary size; all this code would be 
most likely patent encumbered (see [1] - search for MP4), and would only
benefit an unspecified amount of users -- these amount would be necessarily
small, since supporting MP4 would not open new recording possibilities.
Finally, the mkv-to-mp4 conversion can be done in JS using, e.g. handbrake-js
[2].

The not-seekable has been raised before too:
https://chromium.googlesource.com/chromium/src/+/HEAD/third_party/blink/renderer/modules/mediarecorder/README.md#the-produced-recording-doesn_t-have-duration-or-is-not-seekable

[1] https://en.wikipedia.org/wiki/Comparison_of_video_container_formats#Information
[2] https://www.npmjs.com/package/handbrake-js