User blog comment:B214/Don't ask about season 4/@comment-26821288-20161113192209/@comment-3965848-20161113212255

Naturally they have to, only reason they wen't for 3 last time was because the novel was uneven with the anime so they probably wanted vol 11 & 12 in the same season (5th) when they cross that bridge.

Although if they go beyond a 5th, then they'll most likely go for 3 vols again on its 6th (V14, 15 & 16) and for its 8th could go for 4 (v19, 20, 21 & 22), all to avoid the show ending cliffhangers (my guess is).