How do I enable voice control in my LGE Nexus 4 for system commands like "turn on bluetooth", etc?
You mean Google Now, don't you ? For this, you have to click on the microphone of a Google Search bar. You can do this with the one of the home screen. You can also open the Google Search app and doing this with its search bar too. For opening Google Search, do a long press on the Home button and then swipe to the circle with the Google logo. Google Now will be opened. You can also use the app shortcut (in the Launcher or anywhere else).