You need to investigate Proxy files guys. Transcoding the original footage and then rendering THAT footage out introduces another layer of processing to the media and you're cooking it twice.
Many programs support it but I'm only really familiar with it in Premiere.
Before importing your media, go to your project settings and locate the ingest settings. There, select 'create proxies'. Select a proxy format that suits being careful to select one that has the same pixel aspect ratio to your 4K UHD of full fat 4K source material. Then import your media. Put the kettle on because you need to let your system do some work to save you time later. Media Encoder will open automatically and create lower resolution files which mimic your originals in every respect except they're less demanding on the CPU etc. You can pile effects and LUTs and Lumetri on your project and see results in realtime in totally acc eptable quality to get the edit done. Once complete, when you come to render out, the system will automatically return to your full resolution files and give you a fabulous end result. I use this less and less now that I have the new MacBook Pro but on my old 2013 machine, I regularly edited multiples layers of 4K source material (albeit a lighter Proxy version temporarily) without sacrificing quality in the end results.
Quick tutorial here for Premiere. Workflow is similar in other programs I'm sure. This workflow approach could mean keeping the quality of your original media at expert and also save you shelling out on a new machine.
Work offline using proxy media | Adobe Premiere Pro CC tutorials