Using Control Panel, Hub Info V3, or the reboot app, my C4 (now on 2.4.2.160) accepts the reboot request but then locks up (with no access to the diagnostic tool) until power is removed for ~30 seconds, then restored. I normally reboot weekly and had no issues until recently.
I also bump into restart issues during hub updates. I've done soft resets with DB restores and that sometimes works for a while.