lost file from new version of muse score, it tried to load in version 3 and wouldn't open in 4 where I'd saved it, very annoying. Spent ages creating it. How do I get it back? It now looks as though it's been deleted.

You should have a backup copy and/or autosave. Look in the manual to find back up files. It's the same for both versions.

Your file contains only the header and meta tags, no measures and no notes.
Only a backup file may help ...