The advantage of the 3-4 was it narrowed the rush lane options, put four faster defenders down around the LOS to cut off RBs, and when rushing the passer, a well stocked 3-4 unit could send three DL and any one of four LBs making it difficult for OL to set their pass protection before the snap.
The question of why a 4-man front is partially answered by the current pass happy tendency of the NFL...except half or more of the teams on the schedule could be considered run first teams:
-- NYJ (2), Ten, Bal, Atl, Car, Jax, Hou. Run first (8).
-- Mia(2) is run dependent, McD kept the run game intact when he inherited, and until his QB and receivers get their minds in-sync they will probably run to set up the pass. Leaning run (3)
-- Ind and NO are strong passing teams. Buf is leaning pass with Owens signing. Pass first (4)
-- TB is an enigma.
So I make that 11 leaning run, 4 leaning pass, and 1 who knows?
Of course there is the expectation that this will be another high scoring offense, one that puts the opposition in catch-up mode early - which shifts the defensive emphasis over to pass more often than not. I also have to wonder if Dean Pees spent any time studying Boston College's defense with two large bodies in the middle? It could well be one of the goals for a 40 shift is to protect Mayo and leave him more opportunities to make plays without a blocker in his grill.
The remainder of preseason will be interesting to observe.
For those attempting to project a roster:
-- 40 front teams tend towards more DL and fewer LBs.
-- The Training Camp roster suggests NE is leaning towards larger DEs (Seymour, Warren, Green, Wright, L. Smith, S. Williams, Richard, shopping Carter...)
-- Smaller DEs are essentially 3-4 OLBs to keep 30/40/20/50/60/10 front flexibility. (A. Thomas, Woods, TBC, Burgess, Crable, Ninkovich)
-- ILB will be in more demand for their speed. (Mayo, Guyton, Lenon, Alexander)
-- If the expectation is NE will be in Nickel and Dime packages more consistently, it may be that additional DBs might be necessary...
-- If scoring fast is a priority on offense, depth at WR becomes more important.