The popular double-tap-to-sleep feature has been a staple of custom ROMs for a while. Unfortunately, users of locked devices can't make use of the feature if their device lacks such functionality in its stock ROM.
XDA Senior Member aravindsagar created an Easy Lock app, to replicate the function so that it can be utilized on unrooted devices. The app runs on every device with Android 4.0+ and offers a host of configuration options. To use the application, you need to grant it special permissions. Head over to the forum thread to find more details.