3-4 or 4-3 is not the issue at all and people need to understand that.
What is significantly more important is the gap philosophy:
1 gap or 2 gap for the DL?
Spill or contain?
The irony is you have people on here dying for Leavitt bc he runs a 4-3.
But Leavitt's system requires more size and power. It is a "boxing the ball back" system, where the idea is to use power to force the ball inside. The Sam LB in his system needs to be a big strong dude well over 240 lbs. The 4-3 Leavitt runs is the exact version of the 4-3 that Dono uses when we ran single gap fronts.
Here's another example: the 2012-2014 Denver Broncos were a mediocre defense. They played Jack Del Rio's 2-gap 4-3. Essentially, the DTs were asked to be big powerful space eaters, responsible for 2 gaps. That Temple guy "Potroast" excelled in that system b/c it is a read and react philosophy from the interior DL.
Needless to say, that defense was completely average and downright bad at generating QB pressure.
Fast forward today, and Wade Phillips runs a 3-4 defense. But his defense is single gap, and so they dumped "Potroast", and the DL are now charged with getting up field off the snap. They are quick, fast, and they slant and stunt all over the place. B/C of their alignment, they are able to send 5 on every play if needed, no problem. Denver now boasts the top defense in the league.
So you went from a 4-3 2-gap defense that was mediocre, to a 3-4 single gap defense that is elite. Not b/c you changed a goddam alignment, but b/c you changed the defensive philosophy.