Read-only mode for Android? I need a read-only Android test device like a https://help.ubuntu.com/community/LiveCD 

The system should run on the Samsung S4 and not on a CD.

Is there a way to set the android system in read-only mode?
In Lollipop, there's Guest Mode

(source: androidpit.info)

Swipe down from the top of the quick settings panel
Tap your profile
Select add guest to enable guest mode

The phone will look like it originally did without your custom settings, home screen changes, and most other personalisation. Guest Mode is also read-only, meaning no one can mess with the settings you've set (e.g. your ringtone, wallpaper, etc.)
If you add a screen lock (passcode or pattern), users won't be able to switch between Guest Mode and Owner at will.
Do note that even in Guest Mode, users can sign into the Play Store and download apps, though I believe the apps will not be available to other users and are not persistent between sessions.