If all four non-primes can be the same number,
then the answer is:
The four non-prime integers are all 1's.
The greatest common factor of 1 and 1 is 1, and the
sum of the four 1's is 4, which is as low as possible.
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If you mean that they all must be different, then,
1,4,9,25
Their sum is 39, which is as low as possible.
Edwin