Have a matrix
$$ G =\left(\begin{array}{llll}2& 6& 0& 0& 1& 0\\ 3& 9& 0& 0& 0& 1\\ 0& 0& 1& 2& 0& 0\\ 0& 0& 3& 4& 0& 0\\ 1& 0& 0& 0& -5& 5\\ 0& 1& 0& 0& 5& 3 \end{array}\right) $$ I want to get the 4x4 of this matrix and add it to $H$ so
$$ H =\left(\begin{array}{llll}2& 6& 0& 0\\ 3& 9& 0& 0\\ 0& 0& 1& 2\\ 0& 0& 3& 4\\ \end{array}\right) $$
How would I do this with MatLab? I tried looking through there help but I couldn't seem to find it. Thanks in advance and sorry for the horrible formating.