I'll give this a shot tomorrow. Thanks!

MM5 will offer to re-import if you close MM, and then delete the MM.DB file from MM5. ... Also delete the persistent.json file if you also want to clear settings.
If you are installing as "portable", both files are in the MediaMonkey 5\Portable folder.

nb: sometimes the main window of MM5 sits on top of the offer to reimport the db, so if it opens empty look under the main window.

I installed MM5 to play around with the Chromecast functionality and see what new hotness was on the way, and everything worked better than I expected. It imported my MM4 database smoothly in spite of being enormous (~95k tracks), and it cast without any issues at all. It still crashes semi-regularly (all auto-reported!), and there's still a bit of functionality yet to go, so I'm still using MM4 for my day to day needs, but I'd like to at least open MM5 to cast to the house stereo from time to time. And tinker around for testing in my spare time just to make sure it won't choke on my library by the time it's released.

Is there any way for MM5 to re-import the database from my MM4 install after install, or is the only way to keep them in sync to uninstall and reinstall so the installer will re-import? If not yet, would it make sense to update the installer with a 'modify' option to let me do that? It would certainly let me do a lot more testing.