Lets say I’ve got a mikrotik with all ports bridged together acting as a switch. All data is untagged packets. On 1 port only I’ve got a device which I want to put on vlan ID#3 so that all traffic entering that port becomes vlan 3 but the rest of the ports on the mikrotik remain un altered.
I would configure one port as a master, and have the other 3 ports set as its slave - this would be faster than bridging all 4 ports.
Then the last port I would configure on its own and add a switching rule to add the VLAN ID#3 tag to it. I’m not sure if you can bridge the port with the master port though.
Just one question, what do you want to do with a single VLAN on the switch? Do you have the router connected to another device on another port that also has VLAN ID#3?