After installing the beta 4, some settings were changed. Apparently set to default.
This affected, among other things, display options such as PS/SQL clipboard, search bar, use of a ribbon instead of a menu and the taskbar configuration.
Can something like this happen if the installation is performed for all users instead of for the current user?
I'm not sure if there were any changes from the last install. Anyway, even 'reviving' the beta 3 didn't restore my preferred settings. So then installed the beta 4 again and made all changes manually.

I just noticed that a second launch shortcut for PLSQL Developer (64 bit) Beta has been created on the desktop. Maybe it will help narrow down the problem

An update or upgrade should not affect your preferences. They are all stored in your Windows profile, which is not affected by installing or uninstalling.