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.
Deck over concrete patio. If this deck is going to be right on the concrete patio that you say is 6-8 inches thick and in good shape, I would use pressure treated ground contact 2X sleepers right on top of the patio. I would not attach the deck to the house in case there is any patio/deck movement.