Personally, I didn't retire my C-4 when I got the C-7. I had observed that my C-4 would become slow and nearly unresponsive at times, and that some of my network-based integrations were taking up a lot of the hub's CPU. So now my C-7 handles the devices and the bulk of the automation, while the C-4 segregates all the network-only "devices" and the automations that primarily concern them.
I wrote Filtered Device Mirror to help further reduce the load on the C-7 from network devices on the C-4.
