How many return vents should an ac have going back into the blower?

You should have only a single Return vent. that return should be the proper size for the HVAC unit. The measures of the return look correct. The 4 Ton unit seems light for a 1800 sq.ft home. It's not true by adding another return you'll get more volume. You'll need to look at the output runs of the ducting. They need to be as straight as possible and as short as possible. Look for sharp bends, bunched areas, breaks and disconnections. Everything should be free of holes and tears and hung OFF the ground with proper strapping.