The cameras need to be configured individually with a static and routable IP to access them. If they're behind a router, you would need to access the administrator interface to forward the ports.
Example:
External IP: 100.100.100.100
Camera 1: 192.168.2.1
Camera 2: 192.168.2.2
Camera 3: 192.168.2.3
Camera 4: 192.168.2.4
Access the Port Forward and assign ports for each of the cameras in their configuration interface, then forward each IP to the port for each camera to look like this:
Camera 1: 192.168.2.1:101
Camera 2: 192.168.2.2:102
Camera 3: 192.168.2.3:103
Camera 4: 192.168.2.4:104
After this process, the cameras can be accessed using the External IP address with the each port.
Camera 1: 100.100.100.100:101
Camera 2: 100.100.100.100:102
Camera 3: 100.100.100.100:103
Camera 4: 100.100.100.100:104