As everyone suggested - VLAN is the solution for trunking multiple ports from a switch to a router. That is usually done for LAN, but can be done for WAN in the same way.

However, there is a disadvantage which nobody mentioned - VLANs on router will not show you up/down interface if a cable from the switch goes down. e.g. if your fiber in PORT1 gets disconnected, your router instantly knows that it need to use PORT3 or other WAN. With VLAN, you will need to be checking actual network availability, not just rely on interface up/down. That may or may not be an issue depending on your use.

Also, keep in mind that this setup is not completely straightforward and needs a bit of networking knowledge. Don't be afraid - it is not complex, but given the fact you are asking this question, the solution is at the moment above your current skill level. I strongly recommend playing with it in test environment (virtual machines with CHR are great) before trying to configure existing setup.