Thanks Ed. That's what I was thinking, but only after I saw a picture recently of a 9" Malnati's pizza that someone posted on here a while back (BTB, I think). Before I saw that picture, I figured they cut 'em into 6, which is what I've been doing with the 9" Malnati's clones I've been making lately, except for the one I made last night, which I cut into 4 after seeing that picture. (I've only had Malnati's once, and we got a 14" pizza.)
I prefer a 9" pizza cut into 6, myself, but I can think of a few good reasons why they probably cut them into 4. Mainly, cutting a pizza into 6 slices is awkward and requires some extra mental energy to create equal-sized slices.
I didn't think about this until after my original post, but I have the same question about Malnati's 12" deep dish pizzas. How many slices for a 12"? (You might as well thrown in the answer for the 6", too, even though I figure these are cut into 4.)