Use your editor of choice with superuser privileges to edit the /ect/pam.d/gdm-password file to add auth sufficient pam_pwdfile.so pwdfile=/etc/custompinfile to the top of the file, similar to the following: […]

Source: Setup Ubuntu for PIN login

I’m not sure why this isn’t available in settings, but it should be. I’m under the impression that the normal password is still required for sudo and other priveledged actions.