If your subnet is 255.255.255.0 (24 bit), then the first three octets of all devices in the subnet have to match.
192.168.1.x, 192.168.2.x, etc.
(192.168.2.0/24 and 192.168.3.0/24 respectively)
A device on internal3 would need to have 192.168.2.x, and internal4 would need to have 192.168.4.x. Default gateways would be the Fortigate IP address for each interface.
Please look into a rudimentary online networking tutorial. It will help you loads going forward if you plan on pursuing this as a career.