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That really weird that the 4640 would give that message. As you noted, it doesn't run at USB voltage levels. While it's a bit over 5 years old, I wouldn't expect it to have issues. Mine is still working fine. It's possible that the wall power cube is failing. Roku lists a new power supply for $10, however it doesn't list the 4640 as supported. It lists all Ultra models newer than the 4640, so there's probably a change in the unit. Perhaps @RokuDanny-R can assist.
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Although this may be a 5 year old product, I have only had the device since bought new in July, 2020. So now every time I watch it I get the error message across the bottom of the screen, usually blocking all closed captioning.