1996 Chevy Lumina Won't stay running

I put some gasoline in my tank, which was empty, approx 4 gallons. The next day I drove the car 4 miles to a client, and upon leaving, the car would not stay running. It would turn over, but knock and sputter horribly. I had to get it towed home. I have tried putting a couple of gallons of better, high octane gasoline in, but no change. It will turn over and sputter and knock, then die when I press on the gas pedal.

Fuel pump? Fuel filter?
David