I'm going to experiment. I use the same codec, and render Quicktime .mov files on my MacBook.Thanks breh.
Yeah I shoot everything in 4K 60fps (Unless I'm going for the super slow mo effect like I did with the mustang peeling out, then I shoot at 1080p 120fps).
I export everything in 4K for youtube and 1080p for instagram and facebook.
Compression is always going to be an issue online, but when you export it, it should definitely look crystal clear on your computer...if not...something is wrong.
I use Final Cut Pro X and usually export in H.264 codec.
Is it possible that you're exporting in MP4?
If the image you're capturing is in focus and looks great when editing, but gets downgraded in the export then I'd say something is wrong in your export settings.
What about your quality/bitrate settings?