Saving in MuseScore 4 makes .mscz documents unreadable by MuseScore 3
Although MS4 and MS3 use the same file format extension, .mscz, the file formats used are obviously not the same, since an MS3 .mscz file opened and then saved in MS4 will no longer be possible to open again in MS3.
This is very bad practice! If the file formats are different, then they should be represented by different extensions – that way, in addition, the original MS3 file will not be erased when saving in MS4 format.
Also, there is no warning from MS4 that a file will not be readable by MS3 anymore. I very nearly lost one file, on which I had spent a lot of work, that way (luckily I had saved a transposed version and could retrieve the file that way).
Bottom line: give the MS4 files another extension and all will be well. Right now all is NOT well.