Every router call it differently. here yours calls it a range where we want only to forward a specific unique port, but that shouldn't be a problem, writing only 4321 there.
In this screenshot, one important thing is the target ip adress, it's your PC internal (local) network adress, inside your local network area, not your external ip adress.
Your local PC ip adress is usually something like 192.168.0.* , it is assigned by your router when your system starts, via DHCP. The port must be forwarded to this specific adress to have it working properly. (Because if you have several pc's, tablets, etc, connected to your router, it's important notifying it to wich computer he has to forward incoming packets landing on this port). That is also why VMoe advised trying to have your PC always getting the same, fixed, local area network ip adress for your pc, because if you ask your router to forward to, say 192.168.0.10, wich has been assigned by your router to the PC, this ip may change when you will further restart your PC, because sometimes your router may assign a different IP, thus your forwarding information would not reffer to the right ip anymore and then it will not work again.
You can usually check what is your PC local network ip using the properties/status from your network connexion icon.