I have built a few of these over the years, but it has been a while. The video card is important, but only if they are rendering 3d or using something like After Effects. The biggest area you want to max out on is a Nice motherboard, a good processor, then 100% Max out every bit of RAM you can put in that board. This is because when you edit video, it is in RAM. Then to render it has to process everything you want done at 4K. The Processor L2 cache, buss speed, and RAM speed and amount are always the bottleneck.
Many of the Video editing machines I built back in the 2010ish times would use the intel onboard video, a 2nd PCI cheap video card I found for free for a 2nd monitor, then spend over 1/2 my build budget on really nice RAM and a motherboard. This may have changed a little going from 1080p to 4K.