The HDMI output on the Startech dock only supports up to: 2048x1152 p60 or 2560x1440 p50 while one of the display ports support up to: 2560x1600 p60 and the second display port support up to: 3840x2160 p30
The 4600 graphics driver does not control the external (USB 3.0) videocard / dock. That runs on the Displaylink chipset. found here: http://www.displaylink.com/assets/product_briefs/NR-123313-PB-9-DL-5910_Product_Brief.pdf
In short, the other displays should be connected to the display ports through an display port to HDMI adapter or cable if the displays only have a HDMI input.
Hope this helps.