... Now installing air bags raises the weight capability of the bed and quite a lot ( up to 4000#) ...
Be careful how you interpret this statement... The vehicle's rear axle weight rating does not increase. Bags simply assist the existing suspension.
The following comes from Firestone's Ride-Rite website:
Ride-Rite™ air helper springs mount between the frame and the suspension of light-trucks, vans, and motorhomes. The heavy-duty convoluted air springs will handle all leveling needs with load carrying capacities between 3,200-5,000 lbs per set*.
Ride-Rite™ air helper springs will:
[*]Level the vehicle front-to-rear & side-to-side
[*]Adjustability 5-100 p.s.i.
[*]Stabilize the vehicle while loaded
[*]Maximize vehicles load carrying capacity
*Do Not Exceed The Vehicle's Recommended Gross Vehicle Weight Rating (GVWR)
This following is from Air Lift Company's website:
Features and benefits of LoadLifter 5000
• Brute strength our toughest add-on bags for your heaviest loads, right up to manufacturer recommended gross vehicle weight rating.
• Rugged 2-ply fabric reinforced air springs are built like a tire.
• Fully adjustable, to keep your vehicle riding level and comfortable, whether loaded, towing or light, on any road conditions!
• Engineered to be easy to install in two hours or less.
• Many popular kits require no drilling!
• Up to 5,000 pounds of leveling capacity.*
• Air adjustable from 5 to 100 psi, for maximum comfort to
maximum load support.
• Easy to install, with full illustrated instructions.
• Quick connecting easy-to-use engineered polymer air fittings.
• Tough, reinforced air bellows for long-lasting strength.
• Life time limited
warranty, no mileage limitation
*Never exceed manufacturer's recommended Gross Vehicle Weight Rating.