Answer
common difference = $-2$
The first four terms are: $2, 0, -2, -4$
Work Step by Step
To find the first four terms, substitute $1, 2, 3, 4$ for $n$ in the given formula.
$a_1 = 4-2(1) = 4-2= 2$
$a_2= 4-2(2) =4-4=0$
$a_3 = 4-2(3)=4-6=-2$
$a_4 = 4-2(4) = 4-8 = -4$
The terms decrease by $2$ units, so the sequence is arithmetic with a common difference of $-2$.