I upgraded to a Roku Express 4k from a Roku Express about a month ago. I have a 5-year-old Sony tv. The Roku Express 4k turns on my tv after about 3 hours. Even if I switch it back to regular tv, the Roku turns my tv back on.
I've checked other boards, but haven't seen anything similar.
Try turning off Settings > System > Control other devices > 1-Touch play
Try turning off Settings > System > Control other devices > 1-Touch play