Folks, you are missing what they have returning at LB for the 4-3 for depth. Harris, Lewis, and Bynum are healed. Fugate has a year under his belt at LB. AJ, Maggitt, and Crump have played a lot of football.
Add in the interior DT's if Big Mac and Sentimore make it back. Couch, Hood, O'Brien, Tugboat, Clark, and Saulsberry are back. Walls, Smith and Miller are seniors with a lot of experience. Taylor is back for DE. These guys are all seasoned or have a redshirt behind them. Most have good experinece in the 4-3.
It's all about the secondary. And even that will be improved with scheme alone by using more 2-deep zone. Don't expect aggressive Chavis type D. Expect a controlled, keep the ball in front of you, bend don't break D that squeezes down in the red zone.
I have zero worries on O. The guys will take to it like fish in water. Most were in spreads going back to HS and Chaney ran it some as well. It's the same zone blocking scheme they used last year only with more quick traps (which is a good thing) and counters. Lane, Neal, DY, Q will all do great in it. The passing game is short, intermediate outs, go type patterns, and deep posts. It is a timing offense. Worley and Peterman will love it and ran spreads in their base offenses in HS.