The 2004 Chevrolet Impala is available as a 4-door, 5-seat sedan. Starting at $21,735, the 2004 Chevrolet Impala Base is powered by a 3.4L , 6 -cylinder engine mated to an automatic transmission that returns an EPA estimated 21-mpg in the city and 32-mpg on the highway. The top of the line 2004ChevroletImpalaPolice , priced at $ 21,735 , is powered by a 3.8L , 6 - cylinder engine mated to an automatic transmission that returns an EPA estimated 20-mpg in the city and 30-mpg on the highway.