First go to Network Security>>NAT>>DNAT/SNAT TAB. Click on New NAT rule. Traffic Source: Any Traffic Service: Click on + symbol to create new service definition with destination port 4265. Traffic Destination: Click on folder icon and drag/drop External (WAN) (Address) object. Nat Mode: DNAT Destination: Click on folder if you've already created a Host definition object for 192.168.1.15, otherwise click the + symbol to create one.
Place a checkmark in the Automatic Firewall Rule box, otherwise you'll need to manually create one to allow the traffic.
Did you want to open the port from external to your internal 192.168.1.15 device or did you want to have your internal device the only device that can access that port to the outside world? If the first is the case than above is the solution. If you want the second then you have to create a firewall rule (and disable or delete the created DNAT rule)
