When used on Apple systems, this docking station can support up to a maximum of three displays. Please note that this docking station cannot support four total extended displays on macOS due to Apple not supporting DisplayPort Alt Mode MST(Multi-Stream Transport). This technology is required for a complete quad display configuration with the UD-3900C4.
If you want to learn more about the difference between DisplayLink and Alt Mode MST or wish to learn more about the MST technology, please check out the articles below.