Since a product in maths means the result after multiplying numbers, you need to multiply 2 prime numbers and get 4.
If we look at the prime numbers: 2, 3, 5, 7, 11, etc, we see that the only number we can multiply to get 4 is 2, or, in other words, the only multiplication of prime numbers we can do to get four is 2*2=4.
