4. Find the matrix $C$ such that
$$C \equiv\left(\begin{array}{ll}
2 & 1 \\
4 & 3
\end{array}\right)\left(\begin{array}{ll}
4 & 0 \\
2 & 1
\end{array}\right)(\bmod 5)$$
and all entries of $C$ are nonnegative integers less than 5 .
4. a) $\left(\begin{array}{ll}0 & 1 \\ 2 & 3\end{array}\right)$