Answer
$322$
Work Step by Step
Substituting $s=4$ and $t=8$, the given expression, $
s^4+t^2+s\div2
,$ evaluates to
\begin{array}{l}\require{cancel}
(4)^4+(8)^2+4\div2
\\\\=
256+64+4\div2
\\\\=
256+64+2
\\\\=
320+2
\\\\=
322
.\end{array}