\left[\matrix{%
{\left[\matrix{a_{11}&a_{12}\cr a_{21}&a_{22}}\right]}%
&{\left[\matrix{b_{11}&b_{12}\cr b_{21}&b_{22}}\right]}\cr
{\left[\matrix{c_{11}&c_{12}\cr c_{21}&c_{22}}\right]}%
&{\left[\matrix{d_{11}&d_{12}\cr d_{21}&d_{22}}\right]}\cr%
}\right].