4 individual pipes will make more power. The Hal Hunt pipes "tuck in" which will allow you to get rid of the exhaust "humps" on the lower cowl as well as not needing a special opening in the cowl for its exit. If you visit my website, you will see Hal Hunt pipes on my Longez (as well as a very much simplified baffling system if you go to downdraft).
You can also check out berkut13.com for a slightly different 4 pipe exhaust and a very nice baffling for updraft cooling that is far superior than plans. If Dick Rutan and Mike Melville are not using berkut baffling, they are using something that looks VERY similar.