For reference, internally the UD-3900C4 combines two different technologies in order to drive the attached displays.

The 'HDMI 1' and 'HDMI 2' video output ports are driven by the DisplayLink technology within the dock, specifically a DisplayLink DL-3900 chipset.

The 'HDMI 3' and 'HDMI 4' video output ports are driven by the video output capabilities of the host computer (known as VESA Alt Mode video) in conjunction with a DisplayPort Multi-Stream Transport hub (aka DP MST) within the UD-3900C4.

In brief, in order for both the 'HDMI 3' and 'HDMI 4' video outputs to function the host computer must support DP MST. 

If you are connecting displays to the 'HDMI 3' and 'HDMI 4' video outputs of the UD-3900C4 and things are not working as expected, this may because the host computer does not support DP MST. 

An important note, as of this writing Apple computers do NOT support DP MST. As a result, when the UD-3900C4 is connected to a Mac the video output shown via the 'HDMI 3' and 'HDMI 4' ports will be duplicated or mirrored.  

Again for reference, Plugable has made a video that demonstrates the Apple Mac limitation in regard to DP MST here --> LINK