iMovie Formats
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I thought that it might accept still photographs with a resolution 3840 by 2160 pixels but this is not the case. I also tried with all sorts of sizes but the only time it recognises that you may wish to render in 4K is to start with a movie clip of this resolution.
So if you want to start your movie with a still image first drag a 4K movie clip to the time line then drag the still image in front. That is enough to trigger iMovie to allow rendering in 4K.
Guess who made a movie over a couple of days using still images carefully formatted to 3840 x 2160 with inserted 4K video clips then wondered why it wouldn't render in 4K? Don't you just love the Apple way of doing things?