Mike Sando has an interesting read on the defensive scheme implications of Dorsey's signing:
Again, I don't think the 49ers are suddenly going to unveil a 4-3 base defense. But they do have flexibility with their personnel. They could be headed in that direction. As Matt Williamson of Scouts Inc. noted when I reached out to him Wednesday, Smith was a 4-3 defensive end coming out of college. Ray McDonald was seen as an up-the-field lineman. The fact that Smith and McDonald have flourished in a 3-4 reflects well on them and on defensive line coach Jim Tomsula. There's precedent for Dorsey to make a transition, too.
On the other hand, Dorsey appears ideally suited for a 4-3 defense. That was the word on him as the fifth pick of the 2008 draft and that remained the word on him heading into free agency. And the 49ers do have the personnel to become more of a 4-3 team on defense should they choose to head in that direction.
"I could see McDonald as a three-technique type and maybe Dorsey as one-technique," Williamson said. "Ahmad Brooks could project as a 'SAM' linebacker pretty easily and Bowman would be great as the 'WILL'. Justin Smith could be a strong-side end, Aldon Smith could play the weak side. They do have the personnel."
We ran a 4-3, in our nickel and a 4-2 in our dime packages on 70% of our D snaps last year.
Sando also mentions that the Pats, one of the original 3-4 teams moved to a hybrid with more 4-3 fronts partially because of personnel, and partially because the 3-4 has become so popular, that it is now easier and cheaper to get 4-3 players in the draft. Its a good read: