With the hiring of new DC Billy Davis, many reports have emerged that his defences are based on a 4-3 under front. This has led to a lot of discussion about how current Eagles players might fit, but also a large number of misconceptions about what a 4-3 under actually is (hint: What Billy Davis describes as a 4-3 under isn't really the same thing).
To start with, it's worth remembering that a '4-3 under' is not an adequate description for an entire defensive scheme, it's merely the alignment used by the defensive line (much like the similar '4-3 over' or Jim Washburn's infamous 'Wide-9'). NFL Coordinators that have made particular use of the 4-3 under front in their schemes include Monte Kiffin, Jim Johnson and Sean McDermott.
With that out of the way, it's probably a good time to look at what the term actually means with a handy picture:
This particular image is lifted from an article here about Jim Johnson's defense during his time in Philly and hopefully serves to illustrate why describing Billy Davis's defensive scheme as a 4-3 under doesn't tell us the whole story (and, is actually incorrect).
By definition, a 4-3 under has 4 down linemen that each play with a hand on the ground and 3 LBs that operate out of a 3-point stance. The DEs are both shaded outside of the tackles, the under tackle (UT) plays a 3-technique, lined up between the tackle and guard, and the nose tackle (NT) plays a 1-technique, between the center and guard. This is something we're all familiar with, because it's what the Eagles have been doing for years, but for some reason people seem to be treating it as a revolutionary new scheme that Chip Kelly and Billy Davis are about to inflict upon us.
As I understand it, Davis intends to use a defensive front that is similar to that preferred by Wade Phillips (as shown below):
This is a 3-4 front (only 3 linemen with a hand on the ground) that shows a lot of similarities to the 4-3 under. The alignment of the DL here shows one DE shaded outside the tackle and the other two linemen occupying the same positions as the NT and UT in the 4-3 under. The only real difference is that the weak-side DE is standing up rather than putting his hand on the ground. This guy will still rush the passer more often than not (like Ware did when Phillips was in Dallas) but has a little more flexibility about which gap to hit because his positioning makes it easier to loop around other defenders and rush from the inside.
With that said, it's possible to run this D using pretty much identical personnel to the 4-3 under. The Texans ran a 4-3 in 2010 and ranked 29th in points allowed, 30th in yards allowed and 30th in turnovers forced. Wade Phillips arrived in 2011, the defense transitioned to the 3-4 described above and finished 4th in points, 2nd in yards and 12th in turnovers. That's a remarkable turnaround and they actually retained a number of the same players. LDE Antonio Smith and NT Shaun Cody kept their jobs and remained in the same roles that they had played in the 4-3. Brian Cushing returned as an ILB, DeMeco came back from his injury and Connor Barwin (drafted as a 4-3 DE in 2009) stepped in at LOLB.
In the draft, Houston used their first two picks on JJ Watt to play the 3-technique and Brooks Reed to play ROLB, replacing the loss of Mario Williams to free agency. Their next three picks went on DBs Brandon Harris, Rashad Carmichael and Shiloh Keo. They also brought in veteran CB Johnathan Joseph and S Danieal Manning in free agency.
Overall, I think the Texans show us a reasonable way that the Eagles could address this transition. Keep Cox at 3-tech, keep Jenkins at NT (whilst grooming Thornton as his eventual replacement), keep DeMeco and Kendricks in the middle, use our first two picks on an impact DE and OLB, rotate Graham and Cole as stand up pass rushers (or as the LDE on passing downs) and bring in a CB and a S in free agency. Job done.