Re: back surgery
The discogram is a subjective test, at least to some extent, and is considered controversial for that reason, and due to the fact that it is so invasive. It depends to some extent on the patient's responses.
Obviously you must have had much more pain when L4 was injected and responded accordingly. Annular tears can be present without the patient experiencing any pain, so perhaps that is true in your case, too. Just because you didn't react strongly when those levels were injected suggests that there is little nerve involvement, not that the disc is undamaged.
The CT scan shows what it can pick up through imaging. Perhaps there is nothing clearly visible at L4 or it appears to have a slight bulge, but due to the location of the bulge it is particularly irritating to the nerve, and is much more sensitive.
The main things a discogram indicates is the patient's reaction when each disc is injected, and whether the disc holds the liquid dye or whether there are cracks and fissures and the dye leaks out.
I know someone who didn't feel anything different from disc to disc -- none of the exam was particularly painful...and yet she had two discs that were quite damaged.
Discograms are often done to reassure a surgeon that he/she will be operating on the disc that is actually causing the pain. Sometimes it is difficult to determine which spinal segment is responsible for the person's pain. For example, in my own case, everyone thought most of my pain was coming from instability at l5-S1, but, when the surgeon opened me up, he discovered my facet joints at L3 were completely worn away, something that was not visible on the MRIs.
It sounds like you need to see another surgeon to see if he/she reaches the same conclusion as your surgeon...or you can just wait. Unless your spine heals on its own, eventually it will become clear where the problem is coming from.