With Roth at LB, It's not too hard to have him Blitz to simulate the 4-3. 4-3's are traditionally better against the run, and stopping the run is generally more important than stopping the pass. The 4-3 was very successful here under JJ and Wannstadt.
With the 3-4, you can put an extra player, a linebacker or safety into pass coverage. With a big linebacker like Roth, it is a little more flexible, as discussed earlier.
Of course Zach was our best linebacker in pass coverage, and now that he's gone, I'm not expecting any Interceptions by these current linebackers. So as discussed, you can take out a Matt Roth on passing downs and put in a Jason Allen, and have 5 dbs. Or you can blitz this safety, Jason Allen, who is pretty fast... or have this guy cover the pass and have a sneaky corner blitz.
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Your premise is a good one though. Our d-line is more talented than our linebackers. I favor the 4-3, so I'd probably use it myself.
With a 4-3, and a healthy rotation of backups, you can tire out an offensive line. Basically you take out Roth, and replace him with Starks. On 3rd downs you have Roth, Ferguson, Holliday, Merling. Merling is really fast, and would make a great 4-3 lineman (when he gains some weight, he'll be a starting 3-4 lineman).
Another thing you can do is have Holliday, Starks, Ferguson, Merling with Roth at linebacker, and simulate a 5-2 defense with Roth blitzing. Try and run on that defense, I dare you.
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That being said... as discussed, our run-defense is ranked #12 right now (up from 31 last year). Stopping the run isn't our biggest problem, and so perhaps the returns from playing the 4-3 would be somewhat marginal.
Preventing the pass is our biggest problem, and I don't see a solution on this roster to fix it. There are statisting on the passing defense like Opposing team QB rating, YPA... last year we didn't give up a whole lot of passing yards because teams could get 4.5 yards per rush against us (I think this is down to 3.1 this year).
However, the Yards per Attempted pass last year was in the bottom 10 for both Will Allen at #1 corner, and Michael Lehan at #2 corner. The opposing QB rating was also in the top-10 in the league(very bad), with a very high completion percentage.
Basically, we have a #2 corner in Will Allen playing #1, and Andre Goodman, a #3 corner playing the #2 spot. So our guys can't cover well, which puts pressure on the safeties. Basically, an opposing team can avoid Y Bell, and focus on Crocker (who is often in no mans land), and expect a pass to be completed.
Something you could do, is demote Will Allen even though he is our best corner. If you have Lehan starting as the #1, you can guarantee that the opposing teams #1 WR will have 150 yards receiving per game, as it is a mis-match. However, with Will Allen at #2 and Goodman at #3, perhaps you can shut down the other 2 receivers.
You can constantly double team this #1 WR with a safety, and perhaps we get lucky that the opposing team avoids throwing into double coverage (like they are taught).
This is a bad solution, but it is creative, and probably our best option. I don't think we'll see it, though.
