You're not too far off there. Most new cars have:
1. An energy-absorbing collapse zone.
2. Front airbags
3. Side airbags
4. Seat belts
5. Automatic engine cutoff which activates when the airbags fire
6. Automatic seat belt release, which activates when the airbags deflate
7. Automatic door unlock, which activates when the seat belts unlock.
8. Satellite phone system which alerts a central office when the airbags deploy.
And no one calls that overkill.
First of all car accidents are much much much more common than self defense shootings. It actually makes sense to have as many distinct safety features as you can cram in a car. I know many many more people who have died in car accidents than have been murdered.
That aside the analogy is still flawed. Each of those vehicle features offer some sort of additional added value or function. Does having gun number three really offer you anything that having a primary, a reload and a bug wouldn't? And does gun number 4 really offer any actual protection versus just having the first three? I would contend it really doesn't.
Your list of vehicle features would be more akin to a person carrying not 4 guns but something like:
A gun
A reload
A knife
OC spray
A flashlight
A first aid kit
A cell phone
The thing about carrying four guns is it really doesn't offer any real world advantage in just about any scenario. Lets look at the common arguments for a BUG and look at what if any additional advantage a third and fourth gun add. People actively seeking out situations where there could be a gun fight (soldiers, swat, etc) do not in my experience carry 4 guns.
Reasons people often state they carry a bug:
1. Two is one one is none: In essence to have a whole separate redundant firearm is something should happen that puts the first out of commission. This could be catastrophic failure, or the gun is somehow lost. It is not hard to imagine a number of different scenarios where those types of things could occur.
2. Ability to access the second gun when the first is inaccessible. For example one's seat belt is buckled blocking access to the weapon carried at 4 o'clock but the pistol in an ankle holster is accessible. If you think that is a dumb example, imagine your own.
3. Carrying the second gun in a way that offers some advantage to the way the first gun is carried. What I often hear is the example of a gun that is pocket carried. This could be a coat pocket or pants pocket. On can go hand on gun in a discreet way not possible with an IWB firearm. One can fire through the coat pocket, etc, etc, etc.
4. As a New York reload. The idea is that instead of reloading one will just draw the second gun. Personally, I would encourage these people to do some reloads (and even malfunction drills for that matter on a shot timer. My belief is that doing so will show most people that a new york reload may not be a better choice than an actual reload. This may depend on how and where the second gun is carried versus a mag for a reload etc. For example transitioning from a rifle to a pistol carried OWB inside of 25M is faster. Play with a shot timer and see what works for you. That debate aside NY reload is a justification commonly given.
5. Arming another person: The idea here seems to be that there could be a scenario where you have both the time and opportunity to give the second gun you are carrying to an unarmed person. Ostensibly the expectation is that the unarmed person will be able to effectively use the BUG.
6. Why not? In essence the argument is often that a very small BUG like an LCP for example is so small, light and easy to carry that it offers no downside.
These are the most common justifications I see in discussions about carrying a bug. It may not be an exhaustive list but it is what I recall seeing most often. Lets look at these justifications and see if a third and then a fourth gun make much sense.
First, two is one. It is pretty hard to imagine the catastrophic failure of two separate guns. Given the rate of failures I have had with my defensive guns it is tough to imagine having two of them both go belly up simultaneously. I could see a single pistol catching a round or otherwise being damaged by some external force, the odds of that then also happening to the second gun? Also I can imagine how someone might lose their first gun, get shot in the hand/arm, trip and fall and drop it moving off of the X or to cover, etc, etc). Again the odds of then losing a second gun seem pretty remote.
Ability to access the second gun either more easily or in a way that offers some distinct advantage. I don't really see the third and certainly fourth guns offering real world advantages in this realm that would genuinely justify carrying numbers 3 and 4. Maybe I am just not imaginative enough.
New York reload: As stated above I think this often is not even a good justification for a bug. A spare mag is IMHO a better idea. Not only for speed but because you are then still in the fight with your primary weapon. A weapon that is in most instances a more capable weapon than the bug. If I run a G19 dry, I can probably be more effective quickly reloading it with another 15 rounds of 9mm than A) storing it, or holding it B) drawing say a J-frame and then having 5 shots in a gun that I am not as good with. If you think you are going to need to fire 30 rounds in a gun fight I'd carry a gun with a decent capacity and a reload not a 3rd and 4th limited capacity gun. The idea of getting to a fourth gun as a NY reload is pretty outlandish IMHO, particularly when one is also carrying spare mags.
I suppose if one adopts the justification of arming other people then each additional gun allows one more person to be armed.
Lastly the why not justification. When you get to the third and fourth gun there probably are costs to carrying them. As has been stated earlier the space and weight would probably be better given to other items. A flash light and knife make more sense than a third gun to me. An IFAK makes more sense to me than a fourth gun. The exact item doesn't really matter and may depend on ones location and lifestyle. The gist of it is that I'd rather carry something I might actually use, because the odds of using a fourth gun are astronomically low.
As others have pointed out there are other liabilities with having four guns. I wont rehash them all here. I will again note that whereas I think the ideas of an aftermarket FCG, or reloads getting you convicted of murder and largely nonsensical. I have personally seen someone carrying what was viewed as abnormally large number of weapons bite them in the butt. I think it affected how the investigating officers handled the situation and I know it affected how the prosecutor dealt with the case. This was not an anti gun prosecutor either. He is a pro RKBA guy who if he has pants on probably has a gun. In sum, doing weird stuff makes you look weird to people.
In sum, do what you want but it seems to be something that has potential downsides and if we are honest offers virtually no upside and certainly not in any kind of situation that is even remotely likely.