Requirements

Buttons should work with either touch or clicks
The pin should be hidden except for the last number
Hardcode somewhere in the code right PIN (which eventually could be read from an API)
The pin is always max 4 digits long
When the user inputs 4 digits the pin is checked
If the inserted PIN is correct the screen displays OK and the screen is cleared
If the inserted PIN is wrong the screen displays ERROR and the screen is cleared

Bonus requirement

After 3 wrong attempts the pinpad locks for 30 secs, showing LOCKED on the display