...to Scott's statement.
Bear in mind that Jon Johannson flies his -4 waaaay over gross with 18 hours of fuel, 50 gallons of that in the rear seat area.
My Air Tractor's gross is around 7500 Lbs but I've flown it at 8500 often. You have to be very careful to allow increased approach and landing speeds and no aerobatics. Slowly work up to the weight you want and don't just put it at 1800 lbs without easing up there.
Johannson is flying near 2000 lbs, I'd guess, but reached that point very systematically.
Regards,