Roku's search will look through all available services and offer results in one place. You don't have to search through each app individually to find what you're looking for. It also remembers your previous searches.
You can plug in any set of headphones in the Roku's remote at which point Roku will mute the TV and allow you to listen to your content without disturbing anyone around you.
If you don't want 4K content, Roku 3 might get you more bang for the buck
Since 4K content is still scarce it might be better to get Roku 3 instead. It's practically identical to Roku 4 except for voice search and 4K support that Roku 3 lacks.