It is generally recommended to render out to image sequence and then combine the audio and images together.
This can be done in Opentoonz but it can also be done in a vast number of programs... or via ffmpeg on the command line...
The basic idea being that we not only will generally use fewer resources but we'll have more options available.
MP4 is an option via Output Settings.
It might be of interest to know that Opentoonz uses FFMPEG to render out a PNG image sequence first before it combines that sequence into the final MP4 video.
So you are rendering sequential images regardless, the main difference is that with fast render you don't have as much control over the options.
It is through the Options button in Output Settings that specific adjustments to various formats (to include MP4) can be selected.
Because of the number of frames you are rendering you may also want to explore Task Rendering as that allows for even more options and can free up resources more quickly than other methods of rendering in Opentoonz.