Addition was built with 2x4 Floor Joists…help

Hi everyone I have a bit of a doozy. My house has a living room addition that was built back in the 70s over an old patio slab. The floors had carpet and have been solid but I’m repairing some moisture while we remodel in the room and discovered the “floor joists” are all 2x4 and supported/leveled by 3 inch wood blocks every few feet. The joists are on hangers at both sides and from what I can tell the blocks are pressure treated, but there’s no moisture barrier or anything and the gap between the ground and floor is only 3-4”. Everything has been solid and there’s not much if any sag. There was 2 layers of subfloor a hardwood and osb so I’m also thinking of downsizing to one there too and assessing the joists. Anyone heard of anything like this?