The bike pictured and video above is what inspired me for the build I am working on right now. I got a 4 into 4 exhaust from Benji's Cafe Racers. It cost about a 4th of the price of those guys in Japan.
(ignore the upper half of the bike - it's not done, yet...)
Check out my build thread:
http://forums.sohc4.net/index.php?topic=22216.225ABout half way down page 10 there is a video of the
first start up. The carbs still needed a major tune in that video (and still need some tuning now as far as that goes) but it gives an idea what Benji's work sounds like.
As for your 4 into 1 goes, the easiest mod is to hollow out the muffler and turn it into a Glass pack (or just replace it with a glass pack of some kind). Glass packed pipes will help give the bike a deeper tone, I have found.