Right, obviously if 4 PSI would not deliver enough volume for a given tip size with the control knob wide open then higher pressure would make a lot of difference. On the other hand if 5 PSI (just an example) was capable of delivering twice the volume needed then turning it down would only have the effect of needing to open the control knob a little more. I know some people have argued that it is volume plus pressure that determines the flame characteristics and of course this is true but the pressure in the mixing chamber is going to be the same for a given volume and flame setting regardless of regulator setting, assuming the regulator is set high enough to deliver sufficient volume. The control knob, while being nothing more than a "choke" valve will serve as a pressure control regulator for the discharge (mixing chamber) side as long as the input pressure remains constant, which it will due to the tank regulator maintaining a constant pressure on the hose. In the interest of safety however it is best to use the lowest setting that produces the right flame without having to open the control knob more than 1/2 to 3/4 of a turn. It also makes it easier to adjust the flame when using lower pressure because the higher the pressure then the less a control knob will have to be opened to make a difference in flow making it much more "touchy".
The reason this is important is that it seems when someone is having trouble welding it is often suggested to use different regulator settings when in fact that is rarely a problem. When using a cutting torch the fuel side is about like the welding setting, it simply requires enough volume to produce the desired flame so set the regulator to a reasonably low pressure that will supply just the right volume without excess hose pressure. The Oxygen side when cutting is a whole 'nother story however and settings for Oxygen regulators for cutting is very important and small changes make a big difference.