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A prime number is defined as a natural number greater than 1 that has no positive divisors other than 1 and itself. Therefore, to identify which number is not a prime number, it's essential to understand the properties of the numbers given.

The number 1 is considered a special case in number theory; it is not classified as a prime number because it does not fulfill the requirement of having exactly two distinct positive divisors. Thus, it is often referenced in discussions about prime numbers for its unique status.

The number 2 is the smallest prime number and the only even prime. It has exactly two distinct positive divisors: 1 and 2. Similarly, the number 3 meets the criteria for a prime number since its only divisors are 1 and 3.

In contrast, the number 4 is composite, which means it has more than two positive divisors; specifically, it can be divided evenly by 1, 2, and 4. This additional divisor excludes it from being a prime number.

Therefore, recognizing that 4 has divisors beyond 1 and itself clarifies why it does not qualify as a prime number, while 1, 2, and 3 either meet the prime definition or are