ARI (Air conditioning and refrigeration institute) has an online database allowing lookups of ARI-rated equipment.
I haven't used HVAC-Calc yet, and an earlier poster suggested considering internal building heat loads. We have approximately 8-10 kwh daily consumption that puts heat into the house that AC has to remove during the cooling season. That's comprised of refrigeration, lighting, computer and media equipment. Most families are probably higher than us since we have only one TV, have laptop instead of desktop computers and CFL lighting throughout. A more typical house might gain 2,000 Btuh from these loads. As we all get more gee-whiz gadgets, these numbers will rise.
There are internal latent loads from people, plants, showers, cooking, etc, as well.
Pay attention to earlier posts about equipment ratings that assume an indoor design temp of 80. Most folks want it cooler, and bringing temps down just a few degrees to 77 or 75 will require substantially more nominal tonnage. The governing calculation is Manual S (equipment Selection)
Consider oversizing the ductwork to allow for a 5 ton unit but go with a 2 stage condenser and a variable speed evaporator blower. A 2 stage 5 ton system would run at around 3 tons most of the time, kicking into high gear only when needed.
Teddy Bear is trying to softly sell you a Thermastor whole house dehumidifier, and that, budget allowing and properly installed, will give the best total comfort by providing dehumidification during times of low or no cooling load. I know I want one when we replace our present house.