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.
Alternately, you may use following steps (SAP2000 version 14) to mesh a 9" thick slab into 4"x4.5" divisions:
1. Draw a 9" thick slab, using normal procedure.
2. Make your one slab dimension (e.g. Left to right/horizontal dimension) exact multiple of 4" (may increase actual length a bit to do so).
3. similarly, make other slab dimension (Top to bottom /vertical dimension) exact multiple of 4.5".
4. Divide 'left to right/horizontal' dimension by 4" to determine required no. of divisions in that direction, say Nh.
5. Divide 'top to bottom/vertical' dimension by 4.5" to determine required no. of divisions in that direction, say Nv.
6. Select the slab and mesh it by dividing horizontal (left to right) length into 'Nh' parts, and vertical (top to bottom) length into 'Nv' parts. The result will be the slab divided into desired size mesh.
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