Strange why did you buy a Router for this setup, why not settle for a switch. If you have a switch then you just needs to connect pc’s to the switch and then ensure that they all have ip addresses in the same scope. ex. 192.168.2.1 – 192. 168.2.6 using subnet mask 255.255.255.248. You can use the .1 address for the gateway.
On the other hand if you have a router you need to define 4 different subnets – one for each port ex. port 1: 192.168.11.1-6 using subnet mask 255.255.255.248 and using .1 address as host ip for the router port 1 and any ip between .2 and .6 as the host ip for the pc connected to the port and using 192.168.11.1 as gateway ip on this host. For each of the other 3 ports do a similar setup using 192.168.12.1-6 on port 2, 192.168.13.1-6 on port 3, 192.168.14.1-6 on port 4 using the same subnet mask as previous and following the same guidelines as for port 1.