Hi, staymac,
It is possible that if you wait longer for iMovie to prepare the movie for sharing that you eventually will see the progress wheel start. A 48 minute 4K movie is quite large and may take some longer rendering time, particularly since you are using the less powerful i5 processor. Also, if you are sharing to Best Quality (pro res) that creates a much larger movie that might take longer for iMovie to prepare. You should see little temp files appearing on your desktop with the movie's name on it as the rendering takes place. Make sure that when you share to file that you have unchecked the Share to Theater button in the sharing box.
If still no luck, try the following:
Sometimes a corrupt preference can cause sharing problems. Try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen the old library to get back to your preferences. Now see if you can share your movie to file.
Also, sometimes sharing issues can be confined to one particular project. Try doing an Edit/Select All, Edit/Copy, of the media in your project and then Edit/Paste it into a newly created file. Then see if you can share to file.
All else failing, as a workaround you can drag the your previously shared movie's icon from the Theater browser to your desk top and it will copy it there just as if you had shared to file.
-- Rich
