MS3.6.2 work shows up on 4.2

Mu4 can read Mu3 files. But not vice versa

MuS 3 files can be opened in MuS 4. This is normal and changes that you have made in MuS 3 are of course also available in MuS 4.
However, if you open the score from the MuS 3 folder with MuS 4 and save it again, you can no longer open it in MuS 3 (or only with MuS 3.7 Evolution).
Therefore I recommend: Use two different folders for the scores of MuS 3 and MuS 4 (this is actually the default setting). If you want to open a MuS 3 file in MuS 4 and save it again, copy it to the MuS 4 score folder beforehand and open it from there in MuS 4.