I was wrong! Dirty upgrade lives on! Now testing with RT-AX88U (main), RT-AX86U, and RT-AX58U AiMesh nodes.
My Configuration: AiMesh Router: AX88U 386.1_alpha4-gd4b7146753 + 2 x AiMesh Nodes: AC86U 386.1_alpha4-gd4b7146753 Backhaul (Priority:"Auto", Type:"Wired"), Cisco SG100D 8-Port Gigabit Switch from Router to Nodes, ISP: 1 Gbps Fibre My Test Exercise with Alpha4: Full Factory Reset of AiMesh...
www.snbforums.com
Just a quick update on this (please see the details in the link above.
I was able to add an RT-AX86U as an AiMesh node even though the RT-AX58U could not be connected, even with all the things I tried as outlined in the link above. The RT-AX86U as a node is superior, btw (vs. the RT-AX58U when it worked on Alpha 3 and now on Alpha 4).
Without needing to stop, disable, or otherwise hamper any scripts (nor physically removing the USB drive either), how I got the 'AX58U to connect was by connecting it to the main router via it's WAN port.
The testing continues but just wanted to report the progress so far.
Lesson learned: AiMesh
should connect wirelessly, but it doesn't hurt to test wired mode either.