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?
I've recently setup my MikroTik to accept incoming VLAN ID#2&3 connections and then bridge them with my local & WAN connections, separately. So that single ethernet port had both untagged and tagged traffic on it.
See my thread:
http://forum.mikrotik.com/viewtopic.php?f=2&t=37508