There are indeed four areas where they are located (two groups aft of the hanger one to port and one to starboard, and two groupings forward of the island, both port and starboard), but each of those groupings are actually made up of five 4 seperate and distinct launchers, each with its own armored encasement.But this 20 small VLS are divided into 4 different locations.
The Mk-57 is a fgour cell launcher and it is meant to be seperate from the cells near it or elsewhere on the vessle, with its own armored encasement to protect from multiple launchers blowing up together when hit. That armored encasement is specifically designed to direct any blast away from the ship and other cells if at all possible.
Now, clearly, if a missile is large enough, with a warhead large enough to impart the strike damage to more than one Mk-57 launcher, then all of those launchers involved would then be expected to have their own encasements help keep other non-impacted launchers from suffering damage or destruction from sympathetic, secondary balsts. That's the idea anyway..