The 4G63 block is made of Cast Iron, by far not the lightest metal in the world.   However Cast Iron is very strong compared to the lighter metals that have been used for blocks (Aluminum).  An Aluminum block would save weight but could not withstand the power levels, and boost pressures that the 4G63 has become famous for being able to withstand.  Other stronger lighter metals exist however you have to remember that Mitsubishi/Chrysler was mass producing these little pocket rockets for a dirt cheap price for the performance.  Other key factors in the strength of the 4g63 are highlighted above.  Notice the Yellow circles.  The crankshaft bearing caps are the items that hold the crankshaft to the underside of the block.  In a non-performance 4 cylinder engine it is not uncommon to have 5 of these that are completely independent of one another.  However the Yellow circles show supports that tie the two end caps together.  This helps to balance and hold the bearing caps together.  Many Honda engines will tie in all of the bearing caps together, one reason why Honda engines have also become somewhat legendary in their reliability.   The Red circles above highlight the oil sprayers located in the 4G63 block.  The oil coolers spray oil on the underside of the pistons to help cool them.  This is relatively common on turbo-charged engines because of the elevated combustion chamber temperatures.