Answer
Not a geometric sequence.
Work Step by Step
In order for a sequence to be geometric, the quotient of all consecutive terms must be constant.
Here: $\frac{a_2}{a_1}=\frac{4}{2}=2$
$\frac{a_3}{a_2}=\frac{8}{4}=2$
$\frac{a_4}{a_3}=\frac{12}{8}=1.5$
Thus we can see that the quotient is not constant, thus it is not a geometric sequence.