Roku tv streaming settings

I don't think there's anything on the TV that you can change.  Some providers have a setting on their website where you can set the quality, but I don't know which ones or how prevalent it is.  Most content isn't even available in 4k, so it really depends on what you're streaming, but I'd guess if 4k is available then that's what you're going to get unless the channel/app gives an option to change it.  Adding another device would probably give you the best control.