Add configuration something like this;
/interface bridge
add name=bridge1234 vlan-filtering=yes
/interface bridge port
add bridge=bridge1234 interface=ether4 pvid=1234 frame-types=admit-only-untagged-and-priority-tagged ingress-filtering=yes
/interface bridge vlan
add bridge=bridge1234 tagged=bridge1234 untagged=ether4 vlan-ids=1234
/interface vlan
add interface=bridge1234 name=vlan1234 vlan-id=1234
This setup will create new VLAN-enabled bridge with single pirt ether4. Ether4 will accept only untagged frames on ingress and will tag them with VLAN 1234 on ingress and untag them on egress. It will drop any tagged frames on ingress. The last config (vlan1234 interface) will allow router to interact with device(s) connected to ether4, e.g. you can run PPPoE server off interface vlan1234.
The whole setup above is meaningless as at the end router still communicates with devices and whole communication (apart from reall short timespan spent tagged on bridge) uses untagged frames.
So basically you want to do something else with tagged frames, but I've no idea what you want to achieve. If you want to mix frames from ether4 (now tagged) with tagged frames with same VID, you have to add ether4 as access port to the bridge already carrying traffic for VLAN 1234.