Which of the following is not a prime number?

A prime number is defined as a natural number greater than 1 that has no positive divisors other than 1 and itself. This means that a prime number can only be divided evenly by 1 and the number itself without leaving a remainder.

In the context of the question, the number 4 is the only one in the list that does not meet the criteria of a prime number. It can be divided by 1, 2, and 4, which means it has a divisor other than 1 and itself (2), demonstrating that it is a composite number, not prime.

On the other hand, the numbers 2, 3, and 5 are all prime. Each of these can only be divided evenly by 1 and itself. For instance, 2 has divisors of 1 and 2, 3 has divisors of 1 and 3, and 5 has divisors of 1 and 5. Therefore, among the options given, 4 stands out as the one that does not qualify as a prime number.