I have done this:
(then click Open) about 4 times now. And yet the last time I clicked on an MP4 file it opened up in Quicktime. It's like OSX is ignoring what I told it to do. When I look at the file with Get Info it says it is an MPEG-4 movie. The default option under "Open With" is Quicktime.
This is aggravating. How do I convince OSX that I really meant it when I checked "Always Open With"?
High Sierra 10.13.4