Who's to say that the 4-3 can't be just as productive? The Bears got to the playoffs on defense alone. How well did the Colts' defense do last year? How highly touted is the Carolina defense? Denver did really well also. The Jaguars defense is pretty good as well. Baltimore is switching to the 4-3 also.
It seems like these teams follow a particular trend. They all have a good pass rush. When a defense can put pressure on the quarterback, it also gives the secondary more chances to make plays.
Chicago...Alex Brown, A. Ogunleye(sp?), & Tommie Harris
Colts.......Dwight Freeney & Robert Mathis
Carolina...Julius Peppers & Mike Rucker
Denver....Trevor Pryce, his LB's, and John Lynch
Jaguars....Marcus Stroud, John Henderson, Reggie Hayward
Now I don't know what our D-line will end up looking like, but hopefully with players like Mario Williams, Anthony Weaver (now at DT), and whoever else is going to be in on passing downs (Peek, Babin, etc.). We may turn out to be a much better defense than many of you are hoping for.