Have you had a chance to check the amount of comitted memory yet?
Try flying at different detail settings and monitor flightsimulator.exe via Resource Monitor. Just as PacificSet mentioned, 4K UHD requires the quadruple amount of pixels to be rendered and will likely drive up memory use in addition to just using ULTRA settings.
I’d be interesting to see how much memory MSFS demands from the OS on a 4K Ultra setting flight over dense terrain. I can’t test that here, since I only have an 2070 Super a no UHD monitor.