Like other Android 4.4.4 KitKat device buttons, you may configure the Back Button on Lock Screen to function when it is pressed long. This feature is provided in the Android 4.4.4 KitKat devices to facilitate the users to lock the Android device right from the lock screen in order to save a bit of precious time of users.
Configuring the Back button to sleep the Android device right from lock screen is easier but if you feel any help, you may see the instructions.
Command Line: Settings – Home > Lock Screen – PERSONALIZATION > Button Actions – GENERAL > Long Press Back Button – Button Actions > No Actions / Next Song / Previous Song / Play Pause Music / Toggle Sound / Go to Sleep.
Instructions
1: Tap the icon of ‘Settings’ app on the Home screen of your Android 4.4.4 KitKat device to open up the ‘Settings’ Screen.
2: At the ‘Settings’ screen, scroll down to the ‘Lock Screen’ under the PERSONALIZATION to open up the settings interface for the lock screen.
3: At the screen of ‘Lock Screen’ interface, tap the ‘Buttons Actions’ to view or change the lock screen buttons actions.
4: At the ‘Buttons Actions’ screen, see the status of the ‘Long Press Back Button’ is ‘No Action’. If you want to change the status, tap ‘Long Press Back Button’. It will open up a popup Menu Window with the following options:
- No Action
- Next Song
- Previous Song
- Play/Pause Music
- Toggle Sound
- Go to Sleep
Further see the current selection is ‘No Action’. 
5: If you want to change the option from ‘No Action’ to some other option, tap the option of your choice. I tapped the ‘Go to Sleep’ to opt this. 
6: It will immediately take me back to ‘Button Actions’ screen. Here, see the status of ‘Long Press Back button’ is changed to ‘Go to Sleep’.
From now onward, whenever you need to go to sleep right from the lock screen, you may use the ‘Back Button’ to sleep your Android device.
That’s All!