I have an app that worked fine -- deployed to both iOS and Android without issues -- prior to the Xamarin-to-VS4Mac upgrade. I hadn't done any mobile development since going to VS4Mac but now trying to run this it won't deploy to Android; still works on iOS. The message "Error: Activity class {<app_identifier/.MainActivity} does not exist". Yet, clearly, there is a MainActivity -- the same one that worked fine when I last ran the app before the VS4Mac upgrade.
Other articles I've found relating to this error indicate it is an issue deploying. I'm deploying to an actual phone (Samsung S7) and I see the message "Deploying to device..." for a few seconds but then it dies with the error message. Checking the phone, the app is not found anywhere. Tried the same thing with an emulator and got the same results.
BTW: I also found that none of my pre-existing emulator devices would work and had to rebuild all of them following the upgrade. Not sure that is related but it seems indicative of something.