The common difference is -4, so the coefficient of n in the explicit formula is -4. Every term is divisible by 4, so there won't be 3 anywhere in the formula.
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-4·1 +8 = 4
-4·2 +8 = 0
-4·3 +8 = -4
-4·4 +8 = -8
The formula -4n+8 reproduces the sequence exactly.
