as is)
4 wired ports and "home" wifi SSID belong to "lan" network (and firewall zone)
"guest" wifi SSID belongs to "guest" network (also)
to be)
3 wired ports and "home" wifi SSID belong to "lan" network (and firewall zone)
1 wired port and "guest" wifi SSID belongs to "guest" network (also)
(use case for wifi guest: well, the usual)
(use case for wired guest: occasionally I have to use a wired device that wants to be alone on a network)
With the standard config on my previous devices I would edit /etc/config/network and split the switch there, creating a new interface and so on and so forth. I've tried doing the same on the 4040 and it failed spectacularly.
@jeff mentioned that it was straightforward so I split the discussion to avoid hijacking the original thread.
If you need more information, just ask.