ghc -e "Graphics.X11.openDisplay [] >>= Graphics.X11.Xinerama.getScreenInfo"
results in
[Rectangle {rect_x = 4080, rect_y = 584, rect_width = 1920, rect_height = 2160}, -- D, DP-4.9
Rectangle {rect_x = 2160, rect_y = 584, rect_width = 1920, rect_height = 2160}, -- C, DP-4.8
Rectangle {rect_x = 0, rect_y = 1920, rect_width = 2160, rect_height = 1920}, -- B, DP-2.8