copy paste from one score to another
I recently downloaded a score that was too difficult for my student. I opened a new score and began copy pasting elements of the old score into the new one. It suddenly stopped working and when I paste into the new score nothing happened. I installed the recent update to see if that would fix it, but when I redownloaded the score it opened in musescore 3 so I can't test it out.
Comments
You can copy from one version to another. Might not be perfect.
"when I redownloaded the score it opened in musescore 3 so I can't test it out"
What do you mean here? An MS3 file should open in MS4. Or, do you mean that when you click on the file that the MS3 program starts and it can't read the file because the file is an MS4 file? If that's the case, then you need to tell your OS to use MS4 to open MuseScore files.
If you have both 3 & 4 installed on your computer, don't depend on your OS to decide for you which version to use - just open the version you want directly, then open the score (eg, File / Open).
But also, I wouldn't be advising copy/paste for this. I'd simply make a copy of the score - the entire file, by using File / Save As - and then just edit that copy directly.