Put yourself in his situation. Imagine you are recounting the story of seeing the plane to someone...you are recollecting seeing it four cars away...you are honestly telling me that you can't imagine yourself using the phrase "I was four cars back" to describe being four cars away (regardless of which way traffic was going).
If that is your claim it is just an argument from incredulity.
This is not what I requested. You, of course, have no obligation to grant any request I make of you, but I ask that you show some decency and not waste my (and others') time with such diversions. If you are unable to explain your own thought process, simply say so. You wouldn't be the first to have such difficulty.
But to answer your question anyway...
[...] you are honestly telling me that you can't imagine yourself using the phrase "I was four cars back" to describe being four cars away (regardless of which way traffic was going).
I am honestly telling you that I can not imagine myself using that phrase in that situation.
ETA: On a line, there are four possible ways Person A can describe their position relative to Object B: 1) relative to the direction that Person A is facing, 2) relative to the direction that Object B is facing, 3) along the direction of travel using Persona A as a reference point, or 4) along the direction of travel using Object B as a reference point.
Although I don't know the specific traffic situation at that exact moment, I don't think that it is entirely unreasonable to assume that James Cissell was in motion at the time of his observation. That assumption, in combination with his choice of words, indicates to me that he was using Option #4 as I described above.
Of the four options I enumerated above, the one that most closely accounts for your interpretation may be Option #2. For Option #2 to truly work, Flight 77 would have to be facing
away, more or less, from Mr. Cissell as it crossed over VA-27 behind him. However, VA-27 runs nearly, but not exactly, parallel to the face of the Pentagon that Flight 77 impacted, and if Flight 77 did impact that face at 90 degrees, as you've claimed, then there would be no "back" relative to the plane; Mr. Cissell would have been to the left of the plane.
Unless you have another explanation, I don't see any reasonable way in which you've interpreted Mr. Cissell's words to mean what you claim they
might mean. Can you do better?