Musescore 4 | MuseScore

MuseScore 4 is indeed opening a new instance of the MuseScore when you open an “extra” score. It does not use tabs anymore for different scores. Tabs are still used for parts.

Not sure what you mean with scores being re-titled with numbers and letters. The best thing you can do, after opening the score in MS4, is to save the file in the MS4 score folder (preventing this to corrupt your old score).

If you can share more information related to nummers/letters, perhaps that can be explained as well.