By 4K, you mean UHD screen Resolution, correct?
If so, you will need plenty of memory, and the time and patience for it to render. Video editing is quite often done on Multi CPU, (Not multi-core) machines for the reason that video editing is very CPU intensive.
I've seen Quad CPU Proliant Servers that are a few generations old selling for under $1000, which would be a good choice for video editing. Generally, servers are much more robust in build quality than standard consumer grade stuff as servers are designed to run at maximum CPU load continuously, while gaming systems are usually designed to run at max CPU load for several hours a day.
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