That seems like they didn't think it out enough. B4 was being built and then disappeared, what, four years earlier? Which means it was probably under construction for at least 2 years before that, since it was basically complete. And that means B1, B2, and B3 all had to be started and destroyed in just 4 years. Along with meaning B1 might have been started very shortly after the end of the war.

I know that JMS wanted it to be called "Babylon 5" for reasons having nothing to do with logic, but that still seems like too short of a time span. He could have easily made the line "Twenty Years after the Earth-Minbari War."