Not sure what your workflow accomplishes that you can't do with a straight FCP workflow. There's no need to mix Premiere Pro into the workflow. You can have 4K sequences in FCP and you can render out a Quicktime movie of the sequence at 4K. You just can't output to a monitor at 4K.
In Color, if you give it a file higher than 4K (like 4.5K or 5K) it will render out 2K files even if you set it to render the project at 4K. There's no getting around that unless you know how to reprogram how Color works.