Question How do I port forward through 4 routers

I don't know where u came up with 4 routers, but ANYTHING out of the WAN port of your Modem, you have no control nor should u care.

In a typical home, you have one NAT device (lets be precise here what we are talking bout). NAT=translates one address to another. Port Forwarding are implemented at the NAT device(s).

Your one needed NAT device is the one in the modem box or if you have a plain modem, the box right behind it, and it translate the Public IP to a Private IP.

Private IP is what you care. Do you have more than one Private IP range? then you may have more than one router inside your home indeed that needs Port Forwarding. Is it intentional to have more than 1 Private IP range? 'cuz 1 is typical, easiest to deal with.
 
It doesn't matter really how many routers are in the path. Does your router get a private or public IP on the WAN port. If it is private then there is nothing you can do. The ISP is doing NAT in their router and even if they could they would not want to deal with port forwarding.

You do not have much option. See if your ISP will give/sell you a public IP address. The only other way is to use a VPN services that will give you a fixed IP. Last I checked nordvpn offers this service but there are likely other VPN provider also. It was $70 per year extra in addition to the normal VPN charges.