Yes, iMovie 10.1.8 with High Sierra supports 4k at 60 fps. I know that the literature says 4k at 30 fps. I am basing my statement on the fact that one poster on this forum reported that it edits and exports 4k at 60fps even though the export option box says only 4k without designating the frame rate. I realize that one poster is the ultimate in small samples upon which to draw a conclusion. But if it works for one it should work for all.
If you are still using Mavericks as your profile indicates, you should update to High Sierra and use iMovie 10.1.8. Also, you didn't mention the age of your computer. The older computers have less powerful graphics cards. The choppiness that you observe on editing might be related to that.
Another thing -- if your clips are being exported from your iPhone X with the new HEVC codec, that your phone will use by default, that might cause a choppiness issue with computers older than mid-2015. In that case, you could try changing the default export setting on your iPhone from high efficiency video to "most compatible" and see if that rectifies the problem. You might want to try that even if you have a newer computer. If you still get the problem, full Hi Def 1080 would be an option. It's not 4k, but still darned good.