Answer
$4,8,12,16,20,...$ Arithmetic.
Work Step by Step
When we observe the sequence carefully, it is seen that the difference between two consecutive terms is the same in the series $4,8,12,16,20,...$
Now,
$\begin{align}
& {{a}_{1}}=4,{{a}_{2}}=8, \\
& {{a}_{3}}=12,{{a}_{4}}=16 \\
& {{a}_{5}}=20,... \\
\end{align}$
Then,
$\begin{align}
& {{a}_{2}}-{{a}_{1}}=8-4 \\
& =4 \\
& {{a}_{3}}-{{a}_{2}}=12-8 \\
& =4
\end{align}$
$\begin{align}
& {{a}_{4}}-{{a}_{3}}=16-12 \\
& =4 \\
& {{a}_{5}}-{{a}_{4}}=20-16 \\
& =4
\end{align}$
Therefore, the common difference is 4.
Hence, the sequence is Arithmetic.
