User blog comment:B214/Volume 22 spoilers/@comment-73.46.6.122-20160819203110/@comment-9595671-20160819210815

Actually, the Rookies Four term comes from the fact that Rias, Sona, Sairaorg and Seekvaira are the only four Devils of their generation who actually train their servants. Other devils simply rely on the power they inherited, trading off servants they deem weak to other.

One could say that Riser may make it the Rookies Five since the one DX story about the Gremory-Phenex Rating Game rematch says he's been training, but I don't think he's actually considered a rookie (Ravel would have been more likely) and therefore doesn't count.