Answer
The sequence is geometric with a common ratio of $-5$.
$s_4=625$
Work Step by Step
Substitute $1, 2, 3,$ and $4$ for $n$ into the given formula:
$s_1=(-5)^1 = -5$
$s_2=(-5)^2=-5(5)=25$
$s_3=(-5)^3=-5(-5)(-5)=25(-5)=-125$
$s_4=(-5)^4=-5(-5)(-5)(-5)=-125(-5)=625$
Notice the the next term is equal to $-5$ times the current term.
This means that a common ratio of $-5$ exists, and thus the sequence is geometric.