Yep, when you're completely stuck on a problem, it's good to sometimes just bash it out.
Try 2, 3, 5, 7 then 3, 5, 7, 11 then 5, 7, 11, 13.
This works well on this problem becuase it asks for the smallest value, meaning we can test from small to big sums to guarentee there isn't a smaller solution that we didn't find.
=^._.^=