put a different colour paint on each of the numbers
when someone puts their 4 digit number in, examine their finger, the resulting colour (say DULUX summer meadow or stagnant pond) will allow you to work out the four digits pressed
( Dai_Marzipan Ain't no party like an S Club party, Fri 8 Jul 2005, 14:07, archived)

This is the most
intelligent idea I have ever heard.
You are a genius.
( baldmonkey a frothy foul-smelling vaginal discharge, Fri 8 Jul 2005, 14:21, archived)