Answer
Common difference = $1$
The first four terms are: $-4, -3, -2, -1$
Work Step by Step
To find the first four terms, substitute $1, 2, 3, 4$ for $n$ in the given formula.
$s_1 = 1-5 = -4$
$s_2=2-5 = -3$
$s_3 = 3-5 = -2$
$s_4 = 4-5 = -1$
The terms increase by $1$ unit, so the sequence is arithmetic with a common difference of $1$.