Four valves heads get more flow area in the cylinder as compared to a two valve system, velocity is not the reason why four are better than two.
Which brings up a good point, if two exhaust valves flow better than one, why aren't they separate into the exhaust pipe or header? If separating the exhaust streams keeps velocity higher then this should hold true and all engines would be built with 8 mini pipes leading to the muffler. Truth is it was tried way back in the old days and port friction actually reduces the flow rate due to boundry wall restriction.
Fact is a single pipe flows better than a dual pipe of the same area due to boundry wall effects, most engineering books can illustrate that fact. If you look at the calculations for piping in any fluid system you will see that a single pipe of X area needs two pipes of larger area to flow the same amount of fluid. This is why you don't see a cluster of smaller pipes flowing liquids or gases in an industrial plant when one large pipe does the job better, its also cheaper!
Fluid dynamics are complicated by many factors and flow area is affected by boundary wall restriction more than one would guess, velocity does influence the equation but it favors the single larger pipe.
Don't confuse what you see at the drag strip where the operating range of the engine is from 5000-8000 rpm and dual headers dumping to atmosphere is totally acceptable from a operational standpoint since there is no need to generate torque lower than 5000 rpm.
Fact is we all live in the real world and most hotrods would do just as well with a larger single exhaust than two, but who doesn't like the look of two big pipes coming out the back of the car as opposed to one? It became a marketing gimmick way back in the 50's because of the look and persists to this day.
I guess what I am saying is there no engineering reason why a dual exhaust is better than a similar flowing single exhaust and there are many reasons why a single exhaust is better from an operational standpoint. Heck even I prefer the look of dual exhausts and the sound too!
But I don't fool myself to think that the flat black painted sleeper with the big block and 3" single exhaust is some slow poke because its walkin softly and carrying a big stick, usually the driver of such a car is some old dude like me who knows that the young guns are looking at him and thinking I can take him cause the car looks slow with that single exhaust...hehe!
Don't let marketing hype and fashionable dual tips fool ya grasshopper, there is always an old timer out there with a few tricks up his sleeve. Now you know one of them.
One day I'll have to tell ya about how we stuffed two mufflers in parallel onto a single exhaust 440 big block just so we could get the same flow rate without making a lot of noise, then again maybe not...I might get thrown out of the old timer club for giving away too many secrets.