If A2 = 1, A3 = 3, then S4=______ ．

Let the tolerance of arithmetic sequence be D, then d = a3-a2 = 2 ∧ A1 = a2-d = - 1, A4 = A3 + D = 5 ∧ S4 = a1 + A2 + a3 + A4 = 8, so the answer is: 8

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