As four your four second gap, if it exists, it's quite possible that Zapruder wasn't continuously rolling the film. As I recall those cameras held about two minutes worth of film; so not much. It's also possible that in some 8mm cameras one had to keep a button depressed in order to keep the film rolling making gaps almost inevitable.
ML/NJ
The standard roll for an 8mm camera is 25 feet long and holds five seconds per foot on each side. One must load the film away from direct sunlight, shoot about two minutes, of which the first and last portions will destroyed in loading and unloading, then take out the film and turn it over--also away from direct sunlight--and shoot the other half. One must then remember not to turn the film over and shoot it a third time.
If the Zapruder film is Kodachrome or Ektachrome (as would seem likely for its vintage), and there is exactly one splice with 25 feet of film on either side, missing footage would only be a factor in assessing authenticity if the amount of 'missing' time at the splice was unusually short. Camera films stocks are different from duplication film stocks, so any 'edits' that appear on a camera film would represent material that was never photographed.