Apple has released its new Apple TV Remote iPhone app, designed specifically to emulate the physical Siri Remote that controls the fourth-generation Apple TV. Because the new Apple TV Remote app controls only the fourth-generation Apple TV, it does not replace Apple’s previous Remote app, which remains in the App Store with a new name: iTunes Remote. Unlike the Apple TV Remote app, iTunes Remote can access iTunes libraries and control older Apple TV models.
However, Apple TV Remote can do almost everything the Siri Remote does, with the exception of volume control. It includes a Siri button so you can issue voice commands to your Apple TV with your iPhone. It also features a game controller mode optimized for playing games.
For more information, see my TidBITS article Apple Overhauls the Apple TV Remote App.