One way to find the GCF of 66 and 4 is to compare the prime factorization of each number. To find the prime factorization, you can follow the instructions for each number here:
Here is the prime factorization of 66:
And this is the prime factorization of 4:
When you compare the prime factorization of these two numbers, you can see that there are matching prime factors. You can now find the Greatest Common Factor of 66 and 4 by multiplying all the matching prime factors to get a GCF of 66 and 4 as 4:
Thus, the GCF of 66 and 4 is: 4