Well the primary switch was made in 2011 due to the shortened training camp because of the lockout. BB said it was easier to teach their nickel front instead of the complex 3-4.
The Pats were primarily a 3-4 defense during the Super Bowl-winning years but now, they only really use it in games where stopping the run is the priority. If you see them in a 3-4 now, it’s because BB sees the running game as the key to the opposition’s offense. Still, it’s a different version of the 3-4 now, as Chandler Jones is a different defensive end that Richard Seymour.
The Pats’ version of the 4-3 is quite different than the traditional 4-3 defenses. Often others are a one-gap 4-3, but the Patriots version mixes the techniques along the line. One side of the defense might be in a 3-4, with the defensive linemen two-gapping, while the other side is a 4-3 look with the defensive tackle one-gapping. This is similar to the front the Seahawks run now as well.
The shift to this style I believe is in responses to stopping the pass. It’s a way to get more athletes on the field, since it is almost as much a 2-5 as it is a 4-3. There are more athletes on the field this way and, in theory, they’ll be better to defend the pass.
In a lot of ways 3-4 vs. 4-3 is just semantics. Just because you might suddenly see Ninkovich stand up at defensive end doesn’t mean it’s a 3-4 defense. What defines the defense are the techniques played up front. The old 3-4 in the simple days had three defensive linemen all playing two gaps and the linebackers behind them making the plays.
It’s never this straight forward anymore, outside of 2011 when the Pats were getting killed by Tebow and the running game of the Broncos so they went to this straight-up 3-4.
The biggest thing going forward to to acquire and develop as many versatile linebackers and defensive linemen as possible, with a focus on pass rush. Edge rushers and interior rushers are the priority over old school two-gapping nose tackles, though having one or two of those to plug in the opponents favorite run gap is always nice.