Unique 4 digit number

For unique 4 digit number, the following code work fine

function unique_number($length=10) {

   $string = '';
   // You can define your own characters here.
   $characters = "0123456789";

   for ($p = 0; $p < $length; $p++) {
       $string .= $characters[mt_rand(0, strlen($characters)-1)];
   }

   return $string;
}

print unique_number(4);

Every plugin has extended capability to change the default value of field or form. You should need to know how this work.

I am giving you the example for ACF custom fields.

//for acf
function my_acf_load_field( $field ) {

     $rand = unique_number(4);
     $field['default_value'] = $rand;
     return $field;

}

add_filter('acf/load_field/name=your_field_name', 'my_acf_load_field');