This will depend on if there is a second floor or not, and if it is occupied.
However, in general, 4,000 square feet will take between 48,000 and 60,000 BTU's.
You can check this handy reference guide for cooling requirements:
Commercial Airconditioners, including those designed for large homes, are measured in tons. 1 ton is equivalent ot about 12,000 BTU's. it is not uncommon to have a 3 to 5 ton AC for a large home.
Generally, an Airconditioner needs about 20 BTU's per square foot of living space. Add 5% to cover usage.
If you want a more accurate determination, you can send for a calculation form from:
The Association of Home Appliance, Manufacturers 20 North Wacker Drive, Chicago, Illinois 60606
Ask for the cooling load estimation form. It will include the formulas and is very easy to understand how to put in the information.