You can count by fours by adding four to the previous number.

The numbers that you would count if you started with 0 and counted by fours would be:
0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40 and so on.

The numbers that you would count if you started with 1 and counted by fours would be:
1, 5, 9, 13, 17, 21, 25, 29, 33, 37, 41 and so on.

Notice the pattern when you count by fours.  If you start with an odd number, then all of the counts are odd.  If you start with an even number, all of the counts are even.   Counting by fours is like counting by two except that every other count is skipped.