I use a 4-digit PIN with my bank cards. Each digit can be any digit between 1 and 9. I have set the code so that: • the first and second digits are consecutive (subtracting the first from the second digit gives 1). • the third and fourth digits are consecutive (subtracting the third from the fourth digit gives 1). • the first and fourth digits add together to give the same value as the second and third digits multiplied together.