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. In other words, a prime number can only be divided evenly by 1 and the number itself, meaning it cannot be the product of two smaller natural numbers.

The number 4 can be divided evenly by 1, 2, and 4, which means it has divisors other than just 1 and itself. Therefore, it does not meet the criteria for being a prime number. On the other hand, the numbers 2, 3, and 5 can only be divided evenly by 1 and themselves, making them prime.

2 is the smallest prime number, and it is also unique because it is the only even prime number. Both 3 and 5 also qualify as prime numbers as they cannot be divided evenly by any other numbers except for 1 and themselves. Hence, the selection of 4 as the number that is not prime is correct.