Let Suppose 4 digit number as 4 boxes as shown below.First Box is at 1000^thplace, the Second box is at 100^th place, the Third box is at 10^th place, and Fourth box is at 1^st place.

The 1^st box can be filled with four numbers(2, 3, 4, 5) if we include 0 in the 1^st box then it becomes 3 digit number(i.e. 0234 is 3 digit number, not 4 digits)

The 2^nd box can be filled with five numbers(0, 2, 3, 4, 5) as repetition is allowed.

Similarly, the 3^rd box can be filled with five numbers(0, 2, 3, 4, 5) as repetition is allowed.

Similarly, the 4^th box can be filled with five numbers(0, 2, 3, 4, 5) as repetition is allowed.

Total number of ways is 4 × 5 × 5 × 5 = 500