Sony has released a remote play app that allows you to control your PlayStation 4 with your iPhone or iPad.
With the app you can use your iPhone as a controller (yes, no real keys, but good as an additional controller). You can also use the microphone in the iDevice for voice chat and quickly enter text on the screen of your iPhone or iPad. You also have access to the PS4 at home while on the go.
The app requires at least iOS 12.1 and an iPhone 7, a sixth-generation iPad or a second-generation iPad Pro, and the PlayStation 4 must also have the latest software version.