@stockylobster if the modem is in Gateway mode, you can connect routers to all fours ports if you wanted to, but, I wouldn't want to predict what IPV6 addresses you would end up with for each router and its connected devices.
If the modem is in Bridge mode, you can connect routers to all four ports. Two of those routers will end up with IPV4 and IPV6 addresses. The remaining two will only end up with IPV6 addresses. I don't know how to predict which router will get one or both, other than connecting two routers first so that those routers end up with both addresses, and then connecting the other two routers which should only receive an IPV6 address.
Keep in mind that with the modem in Bridge mode and two routers running, both routers are independent of each other unless you happen to configure a VPN between the two. Without that VPN inter-connect, each network would run independently from the other one. If you wanted to run one router with normal internet access and one router with VPN access only, this would be a way to do it.
Fwiw, with my 4582 running in Bridge mode, I run an Asus RT-86U and just connected an Asus RT-AC68U just to check this out once again. I typically do this for test purposes, run two routers at the same time. The 68U picked up its IVP4 address automatically. I have to check on the IPV6 configuration in the 68U, but don't time at the moment. I usually use IPV6 for test purposes only. I don't expect you to have any issues with either the IPV4 or IPV6 addresses.
If the second router doesn't pick up its addresses after its connected and booted, reboot the modem. Pull the power off of the modem, wait for 10 to 15 seconds and plug it back in. That should resolve the situation.
If it doesn't, you'll have to connect one router at a time;
1. connect and reboot router 1 to ensure that it receives its IPV4 and IPV6 address, then shut down that router or disconnect it;
2. connect router 2, reboot it to see that it receives its IPV4 and IPV6 addresses and then;
3. connect both routers, power them up and reboot the modem.
You shouldn't have to do this, but, in the event that you have any problems, this should work.