I have a Dell XPS with an i5 processor running Adobe Premier Pro and I really can't use 4K. It depends on what you mean by "editing".
My system will play back 4K in real time just fine and cut shots together and export, no problem, but It cannot efficiently manipulate the footage in faster than real time. Also, it can take a day or more to upload depending on the length of the piece.
Editing, at least the way I learned to do it, involves shuttling and sampling and playing both forward and backward in other than real time in order to quickly review and mark the right frame. The editor will simply not do any of that for more than 10 seconds at a time making 4K too cumbersome, too time consuming and too much like lower easier resolutions in final viewing to be worth the trouble, unless a client requires it, of course.
I agree with wytnyt. Old and tired or not, only the eyes of a trained videophile would know how to discern the difference between any HD and 4K. The vast majority of viewers will see only the content given proper exposure.
Am I hearing that additional RAM could help with that, or must one have an i7?