Stage IV

Objective: Generate additional power by pushing a larger amount of fuel-air mixture into the cylinder. 
Modifications: Stage III + Throttle bodies, injectors and air cleaner backing plate.
Pros: Larger throttle bodies and injectors allow more fuel-air mixture into the cylinder - thus generating more power. Larger cylinder volume (by boring or changing them out) is also an option at this stage as is lengthening the stroke by changing the flywheel.
Requires: Engine work (2-8 hrs depending on work) to install new throttle body and air cleaner backing plate (cylinder and stroke work optional)+ Dyno-tune - which is expected to deliver higher torque & horsepower, more efficient combustion (which may lead to better fuel economy), cooler engine running temperatures, and better throttle response.
Cons: Requires extensive engine work. You will never want to ride a stock bike again!

After a Stage IV modification your bike may need a number of additional modifications, not specified in any one stage since it will depend on the specific parts, previous modifications, your engine and the miles on it. These modifications could include lifters, push-rods, valve job, valves, valve springs, cam plates, oil pumps, clutch kit, etc. One modification that is highly recommended after a Stage II is a heavy-duty clutch spring since the power generated will put a lot more pressure on your clutch.