On the OpenWRT Switch page, I have set LAN port 1 (along with a backup LAN port 2 but you can just use a single port) as the VLAN trunk port (tagged) to allow it to carry the traffic through to the VLAN access ports (untagged) [home = VLAN 3 && guest = VLAN 4]. This will create the sub-interfaces eth0.3 and eth0.4 which will contain the separated ethernet Layer 2 traffic from the WiFi clients (ARP, DHCP via dnsmasq, mDNS, etc).
OpenWRT Switch Setup:
Port 1 – Bridged with Tagged VLAN (Home)
Port 2 – Bridged with Tagged VLAN (Guest)
Port 3 & 4 -Trunked with Tagged VLANs (Bridged with untagged management traffic)
UniFi Network Setup:
You can then go to the Networks section in the UniFi AP Site configuration and add a VLAN-Only Network (set the ID to 3 or 4) and then on the Wireless page create an SSID which uses that Network Name in the WiFi settings.
Note: To achieve a similar setup on a OpenWRT AP, you can use the WAN port tagged on those same VLAN numbers and then on the Interfaces page create an unmanaged interface type from the related VLAN sub-interface listed – this interface can then be assigned to the SSID network under the Wireless networks page.