Bill,
Any reflow oven sales guy would be happy to help you with a snazzy sales pitch. I also had some info from paste manufacturers when I pitched it to my boss.
Essentially, I believe, it comes down to control. You may be able to get a 4-zone hot enough to reflow; but with a 7+ zone, you have better control of the heat ramp. When your 4-zone is hot enough to flow the BGA, there's a potential for damage to other components on the board that were spiked too quickly.
We also wanted our 8-zone for lead-free processing. Again, a matter of control (and a matter of having a newer oven that was rated to high enough temperatures to reflow lead-free solder).
I've got a Heller 8-zone, and am pretty happy with it. Vitronics makes a pretty good oven, too, from what I've been told.
Good luck! ..rob
reply »