Solution to blurry playback picture: In your Roku settings, go to "display types" and change to 720.
Background from non-tech guy: I recently cut cable and was disappointed with Roku Express picture quality on my 1080 plasma screen. YouTube TV live programming like NFL games on Fox and NBA on ESPN were particularly bad. So bad, I couldn't see the basketball or numbers on players. You could see the picture trying to resolve and alternate between super blurry and blurry. HBO and Hulu were also nowhere near the HD I was used to with cable. I combed the forum and FAQs but couldn't find anything. Customer service always focused on connection/wireless. My connection was strong and 18" from the router. They convinced me that I'd just have to get used to it. The problem turned out to be the signal from the station. My Roku was continually trying to bump it up to 1080, but the signal from the station wasn't strong enough or had too much traffic (not a tech guy) so it was horribly blurry. When I switched to 720 (Roku settings>display types), the picture improved dramatically. For the first time, I now have HD and it looks as good as cable. I have to believe that 4k is having the same issue. You might have a 4k or 1080 screen, but if the signal isn't strong enough, it doesn't matter.