In general the "experts" consider Roku to be the best device amongst the common ones available (Fire, Chromecast, etc.). I have a cheap Roku hooked up to my Samsung TV as the built in smart app pretty much sucks. The Roku is vastly better.
As for 4K, that is a more complex issue. Do you have a 4K TV. If not, there is really no point in caring. Do you have access to sources for 4K signals? This is growing in availability, but if you do not have a streaming service which provides it, or a 4K deck and discs you again need not care. Finally, and more subjectively, are you really discerning enough to see the difference between 4K and regular HD, and if so do you truly care? I can answer none of these questions for you, and can only pose them.
As for 4K with Roku, a quick search showed me that at least the higher-end Rokus, like the Ultra, do provide 4K for you . . . at least according to what Roku says. It should be simple enough to do more extensive research into this. Google is your friend, and Bing is good too.
