Quality of exporting ...
And what'sthe difference between Apple devices 4K and Apple 1080p? Seems also very small?
iMac, macOS Sierra (10.12.6), iPhone 7 (version 10.3.3)
Did you shoot in 4K?
Do you think your audience will have 4K displays in which to watch your wonderful movie?
If the answer to any of the above is "no", then there is no advantage in sharing your project in 4K, or even making the project 4K to begin with.
Even if you shoot in 4K, you may get better results by editing in 1080 and using the extra pixels to reframe in post.