Answer
See below, $d=3$.
$4,7,10,13$
Work Step by Step
1. Based on the given expression, we have $s_n-s_{n-1}=3n+1-(3(n-1)+1)=3$, thus this sequence is arithmetic and the common difference is $d=3$.
2. The first four terms are $4,7,10,13$
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