If the house is well insulated, a 4 ton unit should work pretty well in about 1800 sq feet. But if the insulation is not good enough, then you need to go with a larger unit.
600 square feet.
1500 sq ft.
around 400 SF, not ft
An 800 square foot area need a 15,000 BTU air conditioner to properly cool. Properly sizes the AC unit ensures that the unit will not be overworked.
One ton of air conditioning can cool approximately 600 square feet of area, if the climate is not too hot and humid. Using that formula, a four-ton air conditioner can cool about 1600 square feet of area.
When referring to tons in relation to air conditioning, it is a measure of cooling capacity. A 3 ton air conditioner has the cooling ability of 3 tons of ice. A general rule of thumb for typical construction is 500 square feet of floor space per ton, so a 3 ton unit could cool a 1500 square foot home.
It depends on the climate, insulation, and layout of the room. In general, a 1 ton air conditioner is suitable for a room between 400-800 square feet, so it may struggle to adequately cool a room of 1800 square feet. It is recommended to use a larger capacity air conditioner for a room of that size.
An 800 square foot area need a 15,000 BTU air conditioner to properly cool. Properly sizes the AC unit ensures that the unit will not be overworked.
According to the Energy Star chart, 12,000 BTUs will cool a room of 450 up to 550 square feet. I'd stay on the low end of that.
900
This size unit is enough to cool a room 450 square feet which gets a lot of sunlight; air conditioners are rated by BTUs not degrees of cooling, you run it until the room is the temperature you have chosen.
I currently have a 5k btu air conditioner and I've found that it'll do well to cool a single room, providing the doors are closed most of the time. To expect much more from it as far as cooling wouldn't be realistic. One of the purposes of an air conditioner besides cooling, is to also remove humidity, and if you use it in a larger area, it'll help some, but you can't really expect a lot of cooling from the unit. It'll also run the compressor continually and the electric bill will probably be quite a bit higher.