Starting with any number at all, I can always reduce it to 4 by applying a consistent rule. 4 is the magic number.

Example: 100. 100 is 10, 10 is 3, 3 is 5, 5 is 4, and 4 is the magic number.

Another Example: 52. 52 is 8, 8 is 5, 5 is 4, 4 is the magic number.

Find out what is the rule?