if i look to your image i think you can also do without vlan if i understand correctly the left side is the router with 4 ports? (I thought it was a switch sorry😋)
What you can do is edit br-lan via luci->network->interfaces->devices tab.
Then you remove devices lan2, lan3,lan4 assuming lan1 is the first port for normal lan.
you will end like this:
note: do not apply only when the interfaces are configurated because you also have to connect back to luci or the settings will rollback especially if your connected port was not lan1 
now you have port 2,3,4 free from lan network
if you go to luci->network->interfaces (click the interfaces tab), you can create a new interface.
you can do something like this:
the gateway needs to be left to empty.
Then you click on the tab dhcp server and then you click setup dhcp server, and voila that port now has its own network.
Also I forgot, you also have to create a firewall zone in the firewall settings tab you can basicly if you go later to luci->firewall point the new created zone the same way how lan was to forward to wan.
is that what you want?, or do you also want to assign such network to wifi?