One of the current hot button issues around the Texans, with the possible impending hire of Wade Phillips, is the transition from a 4-3 defense to a 3-4 defense, which is what Phillips ran most of the time in his recent tenures in both Dallas and Atlanta.
What you need to know is that these days you really can't put a defense into a box. Both the Arizona Cardinals and Baltimore Ravens run hybrid defenses--ones that can show both 3-4 and 4-3 looks depending on the situation. To just focus on whether a defense is 3-4 or 4-3 is semantics at this point. Frank Bush ran some three down linemen sets this year, and should the Texans hire Phillips, they'll run some four down linemen sets next year. What really matters is the skills that the players involved have. Are they limited to pass rushing, or can they also drop back and defend a running back in coverage? Can they only play the run, or are they also pass rushers?
The Texans problem is that they've invested so many picks into their defensive line that even playing the 3-4 part-time is doing lip service to the way they've stockpiled the positions. #1 pick Mario Williams and the #1 overall money he cost to bring in, free agent signing Antonio Smith and the pretty hefty contract for his services, another top 10 pick in Amobi Okoye, Earl Mitchell last year as a third-rounder. Couple this with solid part-time performances by Mark Anderson and Tim Jamison, and the Texans were just fine rushing the passer last year. Had opposing offenses not been allowed their pick of the litter on receiver screens and slants, the Texans pass rush would have been plenty effective. While Smith has some experience in the Cardinals system, none of these defensive lineman is acclimated to the system at this point, and none of them really cover well enough to be 3-4 OLB's.
And that's the real problem with featuring the 3-4 more in my mind. It amplifies one of the Texans biggest need areas: nose tackle. The one spot on the defensive line that didn't look to have a solid player and depth in place is nose tackle, where Shaun Cody has been admirably filling in despite being much better suited to be a backup. Not only does using more of a pure 3-4 potentially yank one of Smith, Okoye, or Williams off the field, it also forces Cody into a role he's ill-suited for: pure run stuffing. All of that defensive line depth? In a 3-4, it's excess. You don't really need three defensive linemen at each position, particularly when two of them are making oodles of dollars and none of them can actually play nose tackle.
I'm not fretting or predicting doom and gloom over a potential switch to the 3-4, mostly because it would actually mean that the Texans are paying some attention to their defense beyond who they can plug into it in the draft. But fixing the Texans defense is about way more than a scheme. As much as I disliked Frank Bush's basic philosophies, the list of coordinators who could have maneuvered around a back four of Glover Quin, Bernard Pollard, (mostly) Kareem Jackson and Eugene Wilson is a short one that is loaded with Hall Of Famers. Of course, the list of organizations who would have willingly gone into the season with that quartet is just as small, but I digress.
If the Texans do land Wade Phillips, I expect that the defense will take a step in the right direction. Phillips is a proven coordinator and has a track record of success in the role. While it's not an optimal solution given the diminishing returns I expect it will find on the defensive line, there are many worse coordinators the Texans could be stuck with, some of which aren't even Frank Bush. But how far this defense can really go will be measured on how it shores up it's weak areas: safety, nose tackle, and a veteran starting cornerback should be the starting point of the roster turnover. If the Texans approach this the same way they approached past Texans offseasons and sit on their hands and wait for the draft, it doesn't matter what system they run.
Could Phillips turn the Texans defense around? He's a decent start. But if the Texans want results, they need to attack free agency and the trade market aggressively, so that they can find solutions to long-standing personnel problems. Only at that point will the record show that they're actually trying to fix the defense.