I don't intend to update the embedded server to use Express 4. There's no good reason that I can think of to go through that.
Upgrades to Express 4.7.2 for "mimosa new" projects will release with mimosa 2.3. 2.3 is done. You can get it from master if you want to try it out. I need to update a whole pile of documentation before I release it and I've been sidetracked by some module development for work. You can see what will be coming with it in the release notes,
https://github.com/dbashford/mimosa/blob/master/RELEASENOTES.mdThere's no guide, per se, but if you do a "mimosa new" for a project in a new directory and include express, you'll get your own Express server and you can use that as a guide.