Now, I'm going to give you every reason that it shouldn't happen. Be prepared for some hard truths.
4-3 defenses are vulnerable to spread offense
In football, things tend to come full circle. There will be a time when the NFL gets back to running sets with two Running Backs, and Quarterbacks will once again run most of their plays from under Center. There will come a time when defenses begin to match athlete with athlete and offenses will be forced to revert back to power sets to counteract it. But as of today, the NFL is at the peak of its pass-happy schemes on offense and defense.
Many NFL teams and coaches are moving to a wide -pen way of playing the game. NFL coordinators have pretty much removed the blocking Tight End in favor of the Slot Receiver and replaced the Fullback with hybrid weapons (RB-WR) to one-up Linebackers in the passing game. Due to this philosophical shift in offense across the league, the 4-3 defense and its offspring (6-2 Defense, 4-4 Defense) are gradually becoming extinct.
The 4-3 defense was built to stop the run and someone finally found a way to control the pass by inventing the Cover 2 - Tampa 2 variation. Teams running the West Coast Offense, a wide-open offensive scheme that relies heavily on short passes in place of running plays, still operated with 2 WR/2 RB/1 TE personnel. However, things change and coaches caught on.
Offensive Coordinators began moving in the direction the game is going now and the inflexibility of the 4-3 defense began to show. Offensive sets with two WRs are the norm now and the 4-3 offers little resistance for this. You see, the beauty of the 3-4 is that zone blitzes, man blitzes and man-zones are endless with an extra athlete on the field.
You continue to keep pressure on opposing offenses with 3-4. This isn't the case with the 4-3 defense. It's a conservative defense. An extremely limited one. Pass-happy offenses pretty much dictate to 4-3 defenses what they are going to run. Against them, there is no real way to dictate tempo of the game in 4-3 defense.
I mentioned in the 'Positive Post' that the young athletes on defense would enjoy the simplicity of the 4-3 defense. Well, the counter argument is that allowing 8-10 play drives with points given up and no diverse way to create turnovers could be absolutely deflating for a young defense.
Jarret Johnson is not a Sam Linebacker
The SAM linebacker is becoming ever more a two-down player; a run specialist that gets replaced when teams go to nickel defense for more athletic and fluid players in coverage.Sam Monson, Pro Football Focus
With teams passing on early downs to get favorable 3rd downs, Jarret Johnson is an absolute liability as a Sam Linebacker if the Chargers were to convert to a 4-3. Sure, Johnson possesses the size (6'4") to engage Tight Ends and see over the top. Yes, it is true that most Sam Linebackers never exceed 10 yards of coverage. However, teams aren't spreading the field to just throw deep. They are spreading out the defense for quick-hitting plays, which almost work the same as running plays. This puts more athletes underneath those areas that the Cover 2 was designed to cover. Ultimately, leading to Johnson having to come up and tackle weapons who at that point have a ton of space to work with.
Frightening Example: Normal Ace 3 WR set by offense: Since Johnson is not quick enough to jam and drop into the hook zone, the Y receiver is now Jarret's responsibility. With Johnson over the top of the Y, the Tight End doesn't get the needed jam that usually assists the Mike, and it starts an avalanche that the defense can't hope to stop.
In order to get maximum production out of the Sam Linebacker, a 4-3 Defensive Coordinator needs the player to be able to cover a Tight End and/or Y Receiver, possible routinely on an island, as effective as they defend the run. The 4-3 defense magnifies this position, the 3-4 camouflages it. In the 3-4, you can get away with a Sam LB that can't cover TEs and WRs, as the DC has more athletes to plug empty holes behind the Sam blitzing or sealing edge.
So, if Jarret Johnson is off the field early, the Chargers would have no choice but to put in...
The Nickel Cornerback
The Nickel is in all defensive playbooks. The responsibilities are practically the same between the 3-4 and 4-3. That's not where I'm headed. My issue is extended playing time for the Nickel.
Whether it's Steve Williams or Marcus Gilchrist, the position is a question mark right now. San Diego can't realistically have high expectations for the rookie and, well, Gilchrist is Gilchrist in the slot. As stated above, three-WR sets have essentially become the base offensive set in the NFL. Slot CBs are basically up against #2 WRs on an every-down basis. They can do this on limited basis, but extending the playing time of the Nickel makes the player an easy target in both the passing and running game.
By installing the 4-3, The San Diego Chargers not only start a huge liability (Johnson at Sam Linebacker) but bring a liability in (Nickel CB) to replace him. Teams that are forced into having to operate out of the Nickel on nearly every play. Coupled that with being ineffective out of base sets, and it's easy to see why the 4-3 defense has become a dinosaur around the league.
Offenses are taught to make you pay and taught to take what the defenses gives them. The players in the aforementioned categories will assure that giving is exactly what the Chargers Defense will be "good at" - at least in the immediate future.
Of course, if the San Diego Chargers can't get production in all facets in a 4-3 defense, then it's only hope solely depends on the creativity of...
The consensus likes John Pagano. The consensus firmly believes that Pagano needs another year under his belt in regard to calling plays. If you, 'the consensus', are correct here, then a switch to the 4-3 does not set him up for immediate success or help him to build on the "progress" made last season.
Logically, play-calling should be easier for a 4-3 Defensive Coordinator, given you get consistent 4 man pressure up front, but there is a twist. Blitz Packages have to be calculated. The decision between when and when not to ask for man-coverage from your CBs can be a humbling experience to say the least.
Being one of the few persons not in the consensus, I see a problem with Pagano and his ability to be creative in play-calling. The 3-4 defense is organized chaos when run like Wade Phillips or Ron Rivera run it. Pagano hasn't followed in their footsteps, and for this I have zero confidence he would be able to find ways to mask the 4-3's deficiencies.
The 3-4 and its athletes allows John to get away with bland calls. Bland calls in the 4-3 will get players easily isolated and will eventually lead to points scored a lot easier by opposing offenses. There is so much the 4-3 Defensive Coordinator would have to do to contribute to the success of the unit.
It's tough enough finding new ways to get things accomplished in one playbook, let alone two. It's easier to get production out of the 3-4 and Pagano should focus on one thing at a time.
If you are still on the fence about either or at the conclusion of this series, just ask yourself: "Do I wait patiently for opportunity or do I attempt to create for opportunity?"