I like it quite cool in my house but the cost is so high. If I buy an energy efficient unit can I save enough to allow me to be as cool as I like it?

  Yes, with change to spare. Let’s assume your 15 year old 4 ton system has an efficiency rating of 6 SEER and you set your thermostat at 78 degrees to save money during the summer months. If you replace that system with a Trane XL19i you can then set your thermostat to 75 degrees and still save nearly $600 per summer. The XL19i’s variable speed furnace will typically save an additional $150-$200 per winter.