Even on my C8, I see high hub load sometimes after an update, and always after a full power cycle. It calms itself down in five or ten minutes and goes away. The issue after a full reset I assume is something to do with the Zigbee and Zwave radios restarting.
I believe for me, it is compounded by having so many custom apps and drivers that initialize all at once. I'm probably living on the edge now of needing a C8 Pro given how hard I drive my hub. Still, the hub is working great once things settle down. I rarely reboot anyway.
So, I can see where a C4 might be stumbling a bit at boot up trying to initialize everything. Have you tried just letting the hub sit for five to ten minutes before trying to use the UI? It could be caused by something related to the latest release for you.
Maybe this?