I'm not up to par on header dynamics, but the biggest thing you have to consider with four exhaust valves opening so rapidly, is that the exhaust gasses are pulsating out. Taking that into account, diameter of piping, length of piping, design of piping, piping junctures all most work together to allow those four flows to merge into one as easily as possible. With the oscillating gasses, you can send them into a bit of turbulence by even a slight gouge in your header. It's quite a science to sit down and design one, keeping all those factors in mind.