Becoming A Rarity: The Boeing 737-400 Advertisement: The Boeing 737-400 is the least common of the 737 Classics in passenger service. This year, it’ll be used by 13 passenger airlines, with South Africa’s Safair the biggest operator of it. Yet, only four million B737-400 seats are planned, as the end of the type nears, except for freight. The B737-400, seen here climbing away, has fewer than four million seats this year. Photo: Getty Images. Boeing 737 Classic aircraft – the -300, -400, and -500 – have almost been consigned to the history books, at least in a passenger sense. Between ...