UHD video sample in 4:4:4
You can use ffmpeg, a free command-line tool, to generate some synthetic videos in the required size and pixel format:

Mandelbrot fractal

ffmpeg -f lavfi -i "mandelbrot=s=3840x2160:r=24" -c:v libx264 -pix_fmt yuv444p -t 5 mb444.mov


This will generate a 5 second UHD video @ 24 fps. You can experiment with the various parameters detailed here, such as coloring methods or amount of zoom.

There's also filters to generate patterns as per Conway's Game of Life or a custom cellular automaton, although those will be very 'noisy' with default settings.

You can also use a test pattern as a source:

ffmpeg -f lavfi -i "testsrc=s=3840x2160:r=24" -c:v libx264 -pix_fmt yuv444p -t 5 test444.mov