To watch the live streams on your PS4 select 'Live TV' from the top menu bar as circled below:
When you select 'Live TV' you'll see all the channels you have access to, select the channel you want to see by using the 'X' button on your controller:
To change the channel while watching Live TV press the up arrow on your controller to show the channel switcher then use the arrows to move up or down the channels. Press the X select button on your controller to change it: