Ethernet Port 4 on XB8 is slow.

Hello - I have a 2 gig plan. WiFi works fine and Ethernet ports 1-3 run at 1 gig speeds. However Ethernet port 4, the only one rated for 2 gig speeds, has recently started outputting a max of 90mbps. I have a cat6 cable and my desktop is compatible with the 2 gig speeds. I have restarted the router and performed a factory reset. Ports 1-3 run at full 1 gig speeds, but port 4 is still slow. Appreciate any assistance, thanks!