Last digit of 5^n is always 5.
Last digit of 9^n alternates between 9 and 1:
9, 1, 9, 1, ...
Last digit of 4^n alternates between 4 and 6:
4, 6, 4, ...
So to find the last digit, we take the product of each last digits,
5 * 1 * 4 = 20 whose last digit is 0. This is the last digit of the product.