If you are connecting displays to the HDMI ports 3 and 4 of the UD-3900C4 and they show the same image or only one display works this may be due to a lack of support for Multi-Stream Transport (MST) DisplayPort Alt Mode by the computer.
One common example of this is Intel and M1 based Apple Mac systems (macOS 10+) not supporting DisplayPort MST. The lack of support for this results in the two displays mirroring the content and behaving as a single display. As such, we would expect these host computers to support only 3 displays when connected to this docking station.
Please ensure that your computer supports USB-C DisplayPort 1.2 or higher.