Answer
$56a^5b^3$
Work Step by Step
Using the $(r+1)$st term of the expansion of $(a+b)^n$, which is given by $\dfrac{n!}{(n-r)!r!}a^{n-r}b^r$, then the $
4
$th term of $
(a+b)^8
$ is
\begin{array}{l}
\dfrac{8!}{5!3!}(a)^{5}(b)^{3}
\\\\=
56a^5b^3
\end{array}