No offense whatsoever to Jack Kirby, but I can't help but think of this line from page 299 of the Comic Book Heroes by Gerard Jones and Will Jacobs (1997)
The conflict [primarily over the art that Marvel didn't return to Jack Kirby in the 1980s] was getting ugly, and Kirby himself was making it uglier. Decades of resentment geysered up from his belly. His memory had always been lousy, and now his rage was twisting it into self-aggrandizing improbabilities. He started claiming to have created things he couldn't have created. He wanted a piece of the income and a "created by" credit for all of Marvel's major characters.
While Kirby has undoubtedly been wronged by and not given enough credit from TPTB at Marvel in the past, I think that some of his claims for creating certain characters should be taken with a grain of salt. Not this necessarily applies to the Fantastic 4 in particular, but might apply to Spider-Man and the controversy over what part Kirby played in that character's creation.