$4$: Given the following two matrices A and $B,$ compute both the matrix$-$matrix multiplications $AB$ and $BA.$

$A=\left[\begin{array}{cc}1& -1\\ 3& 2\\ 5& 8\\ 4& 2\end{array}\right]$ and $B=\left[\begin{array}{cccc}2& 1& 6& -2\\ -1& 4& 3& 1\end{array}\right]$