Intakes, believe it or not can give a motor it's biggest HP gains. It is because the motor is receiving more air at a cooler temperature that this comes about. Exhaust, on the other hand will give you better breathing capability and you will gain more HP as a result. If you look at it from the forced induction perspective you should be able to understand why air intake is so important. If you put on a turbo or supercharger you will see tremendous HP gains. I have never seen any motor get those kinds of gain when it came to strapping on an exhaust. On the other hand the exhaust is very important, in that this is where the spent gases get released. Without a matching exhaust system for any intake system you still loose a little of what you can ultimately gain. In other words, start with the intake side of the motor and go from there. Cooler air temperature, forced or otherwise will produce HP. But if the exhaust is restricted by a stock header and muffler then you will gain HP but not all of it.
On the subject of 4 into 1 versus 4 into 2 into 1 headers both have there advantages.
It really depends on what you're trying to achieve. A 4 into 1 header is good for top end performance. A 4 into 2 into 1 header is good for low end performance.
Most 4 into 1's are for racing.
Most 4 into 2 into 1's are for off the line speed which is where they help to build up torque faster.
These are good for the street because you can tune your motor for top end performance with other methods.
And by the way, TORQUE is the force that gets your car moving.
I hope this clears up a few things.:flash: