If you are interested in this topic may I suggest you read the bibles:
Scientific Design of Exhaust and Intake Systems
Philip H Smith
Four-Stroke Performance Tuning
A Graham Bell
From recent yahoo/io posts:
4-1 systems are very effective over a relatively narrow range. They also have a strong “anti-tuned” frequency which causes “reversion”. This works well in a highly tuned engine where the cam is also tuned to a similar range — though it makes for a “peaky" engine.
4-2-1 systems do not give as large a hit at the tuned rpm but they provide a significantly broader effective rpm range and suffer much less from reversion.
Again, all from dim memory.
Right now I’m struggling with reversion (also known as megaphone-itus) on one of my bikes. I have a beautiful pipe with a 2° taper that works very, very well. Hardly any reversion and great power. Unfortunately, the exhaust volume is so low that it is noisy, very noisy. How does a 140+ db at full chat sound? Glorious but setting off car alarms is the norm not the exception. I also have a 4° tapered pipe with the same top end and much, much quieter due to the larger volume. But it has a simply massive hole a few thousand rpm below peak — like shut it off and park it massive.
All that is why I was very careful when I built my Europa’s exhaust. I use a Banks 4-1, yes, but I situated the muffler sideways for a longer main pipe and, so I can fit a very large, though still free-flow, muffler. Add a relatively unrestricted short super-trapp as an outlet, and my exhaust is both free-flowing and quiet. I also deliberately did not match the tuned frequencies of the header and the cam. This broadens the power band significantly. I have a really fat mid-range (mostly cam) with a nice top rpm hit at 6000 (mostly pipe). Ultimate hp is modest compared to theoretical but it makes for a great street engine.
-----
If you are interested in engine tuning I also recommend Cycle World’s Kevin Cameron. His columns are collected in "Top Dead Center" vol 1 and 2. Even more fascinating is his book “Classic Motorcycle Race Engines”. It covers the history of engine design at the same time as you examine in detail race engines from the 20s on up. Every wonder why engines originally had such long strokes? Why some engines have twin spark plugs? What are the advantages and disadvantages of forged and cast pistons? It’s all there in a detailed but easy to read style.