Not sure if Router problem or not

Strange situation with the router. Only one LAN connection seems to be working, we have two computers both work only when one of the Ethernet cables is connected to the 4th port on the router. It is only this one that works. The internet and connection works on either but only one can be used at a time, if the ethernet cable from one is connected to any other LAN port than the fourth one there is no connection at all, it reads as unplugged.

I am not even sure when this would have started because up till recently I was using a laptop and didn't worry about a direct ethernet connection however it broke and now we're stuck in this problem. I am using a wireless network adapter as a temporary fix but another computer in the house was using it so we have a problem.

Any help is greatly appreciated, thank you.