John,

Are you sure your pool is 30,000 gallons?
720 lbs of salt (plus the 200 estimate for the trucked in water should be good for 30,000 gallons). However, for it to increase to 4600 ppm, would mean that your gallons is....gulp....less, approximately 18,000 - 19,000 gallons.

Another possibility is that your salt chlorine generator needs to be salt calibrated. Please make sure, as PatL said, that you followed the directions for the salt test.