From which spec-page do you take the info, that 2x 4K is supported. [URL]
I have 2 monitors hooked up to Latitude 5410 via WD19S docking station. 1st monitor is 4K, 2nd monitor is FHD. When I hook up only the 4K, it works fine. When I add the 2nd monitor, all displays go to FHD. Display 1: Internal Display Display 2: DELL E2216H (via DP) Display 3: DELL S2721QS (via HDMI)
Can't get 4K resolution on Latitude 5410 via WD19S docking station