You either have two hens who have found companionship (1 more egg will be laid) with each other, or you have a couple, and some other hen has sneaked another egg in the nest.
All 4 eggs may be fertile.
Even though two females have paired up, they usually get some male companionship on the side.
When it has happened in my lofts, I have had good luck with them raising all 4squabs.
Keith
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