Every number is built out of prime factors, just as every molecule is built out of atoms. Prime numbers can be thought of as the 'building blocks' of the whole numbers. Split into a product of primes in just one way (if we don't worry about the order of multiplication).
But 4 is not a prime, because it's divisors are 1, 2 and 4. For example, 3 is prime because its divisors are 1 and 3 only. A number (greater than 1) is a prime number if has no positive divisors (factors) except for 1 and itself.
