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Are you using an older version of Final Cut Pro? FCPX allows me to efficiently edit SD and HD while rendering in the background. It would work the same way with 4K, but your computer system needs to be able to deal with the 4X higher bitrate and compression requirements. Frankly I suspect that editing native/compressed 4k files will leave you unhappy no matter which editing software you choose.
I haven't worked with 4K (yet), but FCPX has a nice proxy media workflow. When you add media to an event a window pops up that offers the option of importing as "Camera Native", Optimized", or "Proxy". You could choose the "proxy" import, edit efficiently, then export in a 4K format. All without purchasing additional software.
Here's a link to a Larry Jordan article with lots more information: