Also have a Sammy 50" 4K. Just got Roku Ultra for that TV and moved the Streaming Stick+ to the bedroom. Upon setup, the Ultra (under Display Type) was telling me it couldn’t do 4K because it was only showing HDCP 1.4 or something. My old “high speed” HDMI cable was apparently not high speed enough. Make sure you have an HDMI cable that passes all of the correct signaling. It should be certified for HDCP 2.2.
Ideally I think it’s 4K@60Hz, 18Gbps (not 10.2), HDR, HDCP 2.2 and I think Mine is YUV 4:4:4
Bought a new cable (actually 2 of them, 6 footers, from Amazon for $12) and everything basically fixed itself.