I have a Vizio 4K HDR10+ TV (V436-G1) so naturally I set the Roku Settings/Display Type to "4K HDR 30Hz TV". That shows 2160p on my Vizio TV display meaning it is receiving 4K. (note, Roku auto gives this same setting)
I just discovered that if I set Display Type to "4K 30Hz TV" (Which I had to do to get something in AsianCrush streaming app to stop frame skipping(jerky video)), It outputs 1080p or 1K to my Vizio. NOT 4K as expected!!!
This is true for my new Ultra LT and my old Premier+ (I plugged the old one temporarily to check it).