How many 4 digits numbers have only even digits?

The first digit can be any one of four (2, 4, 6, or 8). For each of those . . .

The second digit can be any one of five (0, 2, 4, 6, or 8). For each of those . . .

The third digit can be any one of five (0, 2, 4, 6, or 8). For each of those . . .

The fourth digit can be any one of five (0, 2, 4, 6, or 8).

Total number of possibilities = (4 x 5 x 5 x 5) = 500 .