Initially, don't worry about the thickness/depth direction. Add gridlines every 4" and 4.5" in the X and Y directions then quick draw window over your grids to add area shell elements, or if you've already drawn your slab with shell elements, select it then use Edit menu to mesh by gridlines. Next, select the meshed slab and use Edit menu to extrude the areas into solids normal to the area face to create your 9" thick bricks. When extruding, select option to delete the selected areas during the extrusion.. otherwise, you would double-count the shell stiffness and mass with the newly-extruded solid elements.