There's plenty of reasons you might want to take a screenshot on your Nexus 4, be it showing off your home screen, troubleshooting an app, or demonstrating to a friend of family member how to complete a task on the device. Google has made it easy to not only take a screenshot on the Nexus 4, but to also share the most recent screenshot directly from your notification tray.
To take a screenshot on the Nexus 4, press and hold down the power and volume down buttons at the exact same time. After a few seconds you'll feel a vibration on your device and you'll see the screenshot minimize into the notification tray. Pulling down the notification tray will in turn allow you to quickly share the picture.
Should you need to review screenshots at a later time, you can find them in the Screenshots album in the Gallery app.