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if these four numbers are odd then sum of these number is even. So one of these numbers is $\boxed{2}$

If all 4 primes were odd, their sum would be even. Therefore, 2 must be present.

We need odd + odd + odd + even. The only even prime is 2.

To have an odd sum of four numbers, 1 or 3 of those numbers must be even. Since 2 is the only even prime number, therefore there is only 1 even number in that 4 numbers and that is 2.