Looks like LG G4 has some touchscreen issues. According to an XDA thread the device fails to register quick taps and swipes, effectively preventing the Knock On and Knock Code features to work properly.
XDA also conducted a poll on the issue, and of the over 200 participants who’ve voted so far, nearly 60% have confirmed that they are facing the problem. Take a look at the video that shows this issues:
If you want to check your G4 for this problem, just follow these steps: open the dialer and type in 277634#*# (this should open the service menu); head to Device Test -> Service Menu – Manual Test -> Touch Draw Test – Manual; and then touch, tap, knock, and slide on the screen – there will be no red mark if the phone fails to register a particular action.