I have two Roku streaming stick plus and two Samsung 4 k TVs , both show a flashing line when set to 4K for Amazon prime shows that are full screen at the bottom making it unwatchable, if I revert format back to 1080 it goes away but this defeats the purpose , does not occur on Netflix, anyone had this issue?
You didnt mention your Roku's firmware version (Settings/System/About) or app version (highlight the app and press Options (*) on the remote).
This sounds like a Prime app issue; however, try these:
1) If manually configuring resolution, set it to "Auto": Settings/Display type=Auto detect - try again
2) Enable/disable refresh rate matching: Settings/System/Advanced system settings/Advanced display settings/Auto adjust display refresh rate=enabled/disabled (change it) - try again
3) Restart your Roku: Settings/System/System restart/Restart - try again
4) Check/change the overscan/scan setting for your Samsung TV - try again
5) Contact Amazon CS: https://www.amazon.com/gp/help/customer/contact-us