I would not go as thin as 4" on anything that will support the weight of automobiles and even an occasional truck. I would keep the pour 6" thick of more and reinforce well. I pour sidewalks 4" thick, but places that support heavy weight should be 6" thick. Use rebar throughout in addition to wire. Not just on perimeter. I also recommend a footer. I assume you want to put up a pole building if not considering a footer. A structural engineer may be able to give you some alternate designs to a traditional footer. Just make sure what you do is structurally sound. May also consider going 24'x28' or even 24'x30'. If you're going to build it, make sure its big enough. Glad to see you were able to post. Remember, got to make it big enough to hold all your hemi chargers.