Just open the score in MS4. If you have installed MS4, clicking on a .mscz file should open it in MS4. But you can also run MS4 and then open a score from the [File]> [Open] dialogue, or you can drag and drop a score onto the MS4 desktop icon or onto a running instance of MS4
Important note If you save a score from MS4 you will not then be able to re-open it in MS3. The internal file format has changed and is not backward compatible. If you will want to re-open a score in MS3 it is best to save the new version with a new name e.g. MyScore-MS4 for example. That will leave the original score in MS3 format.
Comments
Just open the score in MS4. If you have installed MS4, clicking on a .mscz file should open it in MS4. But you can also run MS4 and then open a score from the [File]> [Open] dialogue, or you can drag and drop a score onto the MS4 desktop icon or onto a running instance of MS4
Important note If you save a score from MS4 you will not then be able to re-open it in MS3. The internal file format has changed and is not backward compatible. If you will want to re-open a score in MS3 it is best to save the new version with a new name e.g. MyScore-MS4 for example. That will leave the original score in MS3 format.
See https://musescore.org/en/handbook/4/opening-and-saving-scores for more information
See also https://musescore.org/en/node/338075#new