User blog comment:HollowIchigo58/High School DxD Season 4 Announced/@comment-50.139.232.230-20161025045141/@comment-9595671-20161025050832

The Satan Ranger part of Volume is canon though, and in Season one only 10 of the 12 episodes were of the first two volumes. The student Council didn't officially appear until Volume 3.

The other two episodes minus the small bit with the Student Council were side stories from Volume 8 that took place around these times, but the Satan Ranger part is between Volume 7 and 9 canonically.

So it actually makes sense to start the 4th season with it, that or the Rias and Sona rating game since due to the tournament, that can still be salvaged into an episode a well, likely as the game before their game with Sairaorg.