1 A retired Regular officer who also is employed by the federal government and is subject to the dual compensation statutes may have that reduction reduced because of the waiver of retired pay to receive VA benefits.
If they don't, then 4 thick is usually standard for a patio slab, how large or small it is doesn't really matter. The most important part is the sub-base. Make sure you have a good gravel base. I would recommend at least 12 inches of compacted gravel. I like gravel where the aggregate (rocks) are no larger than 1.
Adding more concrete. One reason to add concrete to an existing slab is to level or raise the height of the slab. Concrete patios and decks are usually just a poured section of concrete sometimes over a gravel base but usually just onto the bare soil.