Here is another solution if you don't want to buy a high-end system:
Use proxies to edit your videos.
With premiere you can create low-resolution versions of your 4k videos to be used in your edits. You'll be able to edit in real-time with no lag.
Once you're done and ready to export, Premiere will automatically export to a final format using your 4k files.
Its by far the easiest solution if you don't have a beast of a machine to edit raw 4k files. Using proxies, you could edit 4k video on almost any generic computer and its a technique used by most professional editors.
Here's how to enable proxies:
https://helpx.adobe.com/premiere-pro...oxy-media.html