I've configured this as per the solution provided and cannot get the verification to work properly.
Everything looks fine (port-channel up with correct member interfaces, trunking is fine, CDP neighborship between SW1 and SW4), but I cannot get SW1 and SW4 to see each other with STP.
I've created VLAN interface 146 on SW1 and SW4 with IP addresses 220.127.116.11/24 and 18.104.22.168/24 and can ping between them, without any issue.
I'm unable to ping between R1 and R4 though.
I've even reloaded the four switches, but even that doesn't help.
Any idea what to troubleshoot next?