I had the same problem, I tried the extender and it still overheated. They need to put heat sinks on them. But that would dip into there profits if they did that. Before they had the sticks their was never a problem with overheating. Until they came out with the sticks. Now nothing but problems.
I only had a Roku 3 before the stick which did not do 4K only HD. It was always astonishingly hot and ultimately starting having issues or restarting on its own. Heat generally isn't kind to electronics.
I wonder if anyone has tried an Amazon Fire Stick and knows if it has problems. I've never owned one but I've seen the home screen and don't like the interface.