Petkov wrote:

Feynman [wrote], ”It is therefore impossible to get all the mass to be electromagnetic in the way we hoped. It is not a legal theory if we have nothing but electrodynamics” [13, p. 28-4]; but he was unaware that the factor of 4/3 had already been accounted for [10]).

It's worth noting that Feynman's statements are actually correct. According to Wikipedia, the problem is solved by postulating a non-electromagnetic attractive force holding the charged particle together, which subtracts 1/3 of the 4/3 factor, leaving unity. Petkov doesn't explicitly say that Feynman is wrong, but his phrasing might leave that impression.