Unlike iOS 11.3, iOS 11.4 seems to be primarily aimed at fixing all the minor niggling issues with iOS 11 and improving some of the new features introduced in iOS 11.3. Amidst those small changes, Apple is also improving Siri by adding AirPlay command recognition to it. 

Right now, it is not possible to use Siri for AirPlay commands. So, even if you own an Apple TV or HomePod, you won’t be able to tell Siri on your iPhone to play a movie or podcast on your TV or HomePod.

With iOS 11.4 though, Apple will add AirPlay command support to Siri thereby making all the mentioned scenario possible. You will also be able to use Siri on your iPhone to pause any content playing on your HomePod or Apple TV.

The feature is already available on beta 3 of iOS 11.4 released by Apple yesterday.

Do remember that usual Siri limitations continue to apply here. You will not be able to play back songs from Spotify on your HomePod, though you can use the voice assistant to play content from Netflix on your Apple TV. It remains to be seen if Siri on the HomePod and Apple Watch will also gain AirPlay integration or not.

If you are heavily invested in the Apple ecosystem and own an Apple TV and HomePod, this will definitely be a welcome improvement.

Source: iphonehacks