4 into 1's are cheaper to make, lighter, are often tucked up higher/tighter on the bike - increasing cornering clearance, especially over 4/4's
they also usually last longer than stock factory 4/4 or 4/2 systems due to several reasons
a) it was often the welded on original muffler part that rusted out and most 4/1's have alloy mufflers
b) one of the main reasons that the muffler rusted out was due to the relatively low amounts of hot gases passing through each individual pipe, the muffler area would take a longer time to heat up enough to dissipate the condensation that will always collect whenever an engine is started into cold pipes. hence this condensation stays and rapidly rusts out the thin steel mufflers. lots of these mufflers had a little drain hole in the lowest part of the muffler but this usually blocked up with carbon quite quickly
as a 4/1 will have all 4 cylinders worth of hot gas passing through the one muffler the muffler gets a lot hotter a lot quicker, so even steel mufflered 4/1's will rarely rust out (i used to have a Walker Indy full race system 4/1 with a welded on steel muffler that was at least 25 years old when i got it and it still only had a bit of surface rust, i also had a set of orig CB750 4/4 pipes that had been taken off the orig bike when it was only 2 years old and were rusted to pieces)
and to many they do sound better (personal perspective of course)
the performance boost theoretically comes from improved scavenging of exhaust gases due to the gas from one cylinder passing the end of another header pipe at the collector or junction area. this gas creates a partial vacuum in the second header which then draws the exhaust gas out of that pipe and as that pipe empties it creates a vacuum in pipe three and so on (not the most technical explanation i know, but hopefully you get the idea)
