If you have a trunk port between SW3 and SW4, disable CDP, disable DTP by setting nonegotiate, allow only vlan 3 on SW3 and set that as native, and do same for vlan 4 on SW4.
At this point effectively you have an access port, sending only untagged traffic. You will see that STP puts the ports into blocking with a message like:
%SPANTREE-2-RECV_PVID_ERR: Received BPDU with inconsistent peer vlan id 3 on FastEthernet0/15 VLAN4.
%SPANTREE-2-BLOCK_PVID_PEER: Blocking FastEthernet0/15 on VLAN0003. Inconsistent peer vlan.
%SPANTREE-2-BLOCK_PVID_LOCAL: Blocking FastEthernet0/15 on VLAN0004. Inconsistent local vlan.
STP has detected a different vlan each side (assumedly from the extended bridge ID being priority+vlan) and blocked the port. However if you set the ports to be access ports, the vlan mismatch is ignored and everything is fine, despite the same BPDUs being sent.
Anyone know why the disparity in behaviour?