Originally Posted by CIAFT
On the first topic, yes the addition of a TE (or 2) on running plays can create extra gaps, however the 6 interior gaps must be accounted for even without TEs.
On the second topic, contrary to what some might say, in theory, the 3-4 is actually better against the run, while the 4-3 is a better pass defense. The 4-3's main pass rush threat comes from a pair of DE's, pass rush specialists. In the 3-4, that pressure comes from an OLB and either another OLB or an ILB, the line really doesn't typically get a lot of pressure in a 3-4 D. On the flipside, the 4-3 is usually a 1-gap defense with linemen covering only 1 gap. That leaves a couple of gaps to be covered by LBs. IF a lineman can be neutralized by an individual OL, then the gap is now opened to the FB/RB combo on the LB. In a 3-4, the linemen have 2 gaps to cover, which, in theory occupies more OL'men and leaving 4 LBs open to plug the holes instead of 3.
All of this is theory, mind you. In practice, players make plays in both the 3-4 and 4-3. However, on the chalkboard, the 3-4 is a run stop defense and the 4-3 is a pass stop defense.