Unless the contractor had a source of dirt much closer to his job site, I certainly hope that you didn't make an EVEN trade of dirt for a pond. Four acres of ground dug to a depth of 12 feet yields 25,813 cubic yards of dirt. 30% decompaction from being loaded into a truck increases that to 33,567 cubic yards. At $1.00 per cubic yard, truck measure, that amounts to a considerable sum. Considering that the contractor couldn't get the dirt without leaving a pond, I hope you made a good trade.