When it comes to 4 or 6 digit pins, its almost impossible to ensure that no pin has been used before. At 8 digits, you might as well be using diceware anyway.
Nothing prevents users from using 0YYYY or 0DDMM/0MMDD.
Every time some site ridiculously insists I "use a more secure password", I sigh and add "A1!$" to the end of my 32-character alphanumeric random string.