....I got my mavic pro. The footage is butter smooth in 4k and looks amazing. After I put a short video together with Final Cut and export it to my desktop, the video is grainy and jittery...The video is only 2min long.
My video team has the Mavic, Phantom 4 Pro and Inspire 2. I've never seen a quality problem exporting 4k video from FCPX, if done properly.
As a first approximation, you can normally export 4k H264 and it will look very good. It's not usually necessary to export ProRes or 2-pass H264 (what FCPX calls "high quality"). For 4k, FCPX exports about 30 megabit/sec which is about double what Youtube recommends as a minimum.
Try these export settings:
File>Share>Master File>Settings
Format: Computer
Video codec: H.264 Faster Encode
Resolution: 3840x2160
Make sure you don't have proxy enabled, otherwise it will only export in proxy resolution. Proxy is set in the viewer, upper right corner, View>Proxy or View>Optimized/original. It should be set to optimized/original before exporting.
The file size FCPX exports for H264 is not that big and normally adequate for uploading to Youtube or Vimeo. If you want more granular control over bit rate and other encoding parameters, you can get Compressor for $50, which is integrated with FCPX:
https://itunes.apple.com/us/app/compressor/id424390742?mt=12