Ok, I put my 4K stick in an actual 4K TV and it works fine now (goes to sleep and wakes up instead of falling into a coma). Guess I will have to buy another 4K Roku if I want something that is compatible with every TV. In short, get a Roku! I have learned my lesson. I got the Fire stick on Prime Day because it was cheap. Never again.
Same problem here, with a old Sony Bravia TV bought in 2007 and a new Fire Stick 4K bought on this year's Cyber Monday. The stick works perfectly until it is turned off for a long period, in which case it won't wake up and has to be power cycled (short standbys are fine). Tomorrow I'll try to pass the Fire Stick 4K output through a powered HDMI splitter, which hopefully should be better at talking to a pre-HDCP TV. If the splitter doesn't help then I'm ordering a power strip with a switch, so at least I'll be able to reset the fire stick without having to rip cables of things.