The project had two start up movies: our logo, and the UE4 logo.
We want to add a third movie.
The movie (MP4, 1920x1080@60fps) is placed in /Game/Content/Movies, and imported.
The Project Settings… list of startup movies has the new movie added.
I select “Cook Content For Windows…” and wait for it to complete. The shows no errors, and does appear to handle the movie.
When I’ve finished the cook, I restart the editor.
Project settings now shows four startup movies listed: Our logo; UE4 logo; Our logo; UE4 logo.