How to use 1 interface with multiple WAN connection

Hi to all, i just want to ask if i can use the single interface to add multiple wan.. 

its hard to explain but this is our firewall setup. We are using 4 4G sim card router to to use as our internet connection because we dont have fiber here in our location.

Our firewall device is Fortigate 300D and it has 8 port interface, 4 interface is ethernet port and the other 4 port is sfp type. 

Now our user is getting bigger and bigger and im thinking of adding 1 or 2 4G router but there is no more available port in the firewall. Can i connect 1 switch in firewall port 1 and then connect the 2 4G sim card router in the Switch? or you can suggest any option. I really needed your help please