4000lbs * 16oz = 64,000oz
64,000oz / 4 oz per day = 16,000 days
16,000oz / 365.25 days = 43.8 Years
Simply, If someone were to smoke 4oz (1/4lb) of pot EVERY day, it would take 43 years, 9 months, and 22 days to deplete the origional supply of 2 tons.
*edit: Or, at $50 per oz... you could turn...