we know in a 4 digits number, thousand digit can’t be zero
hence, for a thousndth place only 9 digits are allowed to be filled.
hence, for a hundredth place only 9 digits (including zero & besides that digit that is already filled) are allowed to be filled
hence, for a tenth place only 8 digits are allowed to be filled
hence, for a unit place only 7 digits are allowed to be filled.
thus total ways = 9 * 9* 8 *7 = 4536 ways
