Pin code reset?

I have a customer who plans to use the MD4-7 as the main engine monitoring/controller in his cab. He does not want to use a key to start/stop the Cummins engine. He wants to have the operator use a pin code that is factory set at the last 4 digits of the machine serial number to access the machine controls. They encourage the operator/owner to change this pin code to their liking for security. If the operator enters a pin code and then forgets it or of an operator changes the pin code and then leaves the company, the OEM wants to be able to have this pin code reset to the original factory 4 digit code when it was first assigned. They would like to do this by allowing one of their service people to access this reset back to factory function with the use of a factory master pin code protected variable. If this pin code cannot be reset, is there any way an authorized person could display the current 4 digits of this code? The reset feature that is currently implemented on the pin code works great to lock the machine up after use. I am currently using a text parameter as the input to the machine access pin code, I am allowed to display this text parameter with my master pin code, but when we have the operator try to change this text parameter, we see a complete keyboard that allows the operator to enter 4 characters (the pin code only allows us to enter numbers). Entering non numeric characters will force a software update to get the machine working again. (this is what we are trying to avoid). Any suggestions?