Answer
common difference = $\ln{3}$
The first four terms are: $\ln{3}, 2\ln{3}, 3\ln{3}, 4\ln{3}$
Work Step by Step
To find the first four terms, substitute $1, 2, 3, 4$ for $n$ in the given formula; then, use the rule $\ln{(a^n)} = n\cdot\ln{a}$ to simplify.
$s_1 = \ln{(3^1)} = \ln{3}$
$s_2= \ln{(3^2)}=2\cdot \ln{3}$
$s_3 = \ln{(3^3)} = 3 \cdot \ln{3}$
$s_4 = \ln{(3^4)}=4 \cdot \ln{3}$
The terms increase by $\ln{3}$ units, so the sequence is arithmetic with a common difference of $\ln{3}$.