A 4,000 BTU air conditioner has 4,000 British Thermal Units of cooling capacity, or energy production required to remove heat from a room. This is a very small capacity unit. Window air conditioners normally range from 5,000 BTU to 24,000 BTU.
An air conditioner must have 20 BTUs for each square foot of living space. There are other factors that add BTUs required, such as the height of the ceiling and the size of windows and doorways. How many people regularly occupy the room, sun exposure and whether the room is a kitchen are other factors to weigh. As the BTU rating goes up, so do the size, weight and cost of the air conditioner.