MuseScore 4 opens scores in separate windows

I've just installed MuseScore 4 (upgrading from version 3.6.2), and am learning how to use its significantly different interface. Most frustrating issue right now is that opening a second score (either from Mac Finder or from within MS4), starts up a new instance of MS4, with splash screen and another dock icon, and then my additional score appears in this new, entirely separate window, instead of in a new tab in the original window. Is this a bug or a feature? Is there a way to tell MS4 "open another score as a second tab in THIS window," as happened in version 3?