This is basic routing, a function that normally routers perform by default, without any special configuration other than setting the IP addresses on the interfaces.
Assuming the ISP take care of routing the 4 public IP addresses to your router, all you have to do is set the IP addresses on the servers and the router, and configure the router with whatever default route your ISP gave you.
SOHO routers have NAT enabled by default, and it is not needed if you set the IP addresses on the servers, so in this case you would have to disable it.
You also need to configure at a minimum some access list to filter the traffic that can reach the servers in addition to have the local firewall of each server properly configured.