This article explains how to stream to Twitch on a PS4.

For a basic Twitch stream from a PlayStation 4 console, you won’t need much more than the following items:

Streamers who want to add footage of themselves or a spoken commentary to their streams will need to purchase these optional accessories.

The official Twitch app for PlayStation 4, which is separate from the Twitch apps for computers and mobile devices, can be installed in two ways.