A state charges polluters an annual fee of $20 per ton for each ton of pollutant emitted into the atmosphere, up to a maximum of 4,000 tons. No fees are charged for emissions beyond the 4,000-ton limit. Write a piecewise definition of the fees F(x) charged for the emission of x tons of pollutant in a year. What is the limit of F(x) as x approaches 4,000 tons? As x approaches 8,000 tons?