How do I change my settings and chart behavior in Rock Band 4?

You can change settings by pressing the Menu (Xbox One) button or the Options (PlayStation®4) button on your instrument controller, navigating to "Options", and adjusting your choices as you see fit.
The main menu also has a Modifiers menu located within Settings, featuring things like Auto Kick, allowing you to play while the game hits the kick pedal notes for you, and Individual Track Speeds, letting every player run their chart at the speed they feel most comfortable.