-----Description----- 
This task requires writing a Lean 4 method that determines whether a natural number is a power of four. The method should return a boolean value that indicates whether the given natural number is a power of four. An integer n is a power of four, if there exists an natural number x such that n = 4^x. 

-----Input-----
The input consists of one natural number:
n: A natural number.

-----Output-----
The output is a boolean value:
Return a boolean value that indicates whether the given natural number is a power of four. Return "true" if it is a power of four. Otherwise, return "false". 

