User blog comment:B214/Don't ask about season 4/@comment-93.142.12.239-20161128133716/@comment-26821288-20161201193942

Well, I don't really know Japanese myself so I can't really say what it says and what it doesn't, but I think it doesn't say anything about season 4 for two reasons. The first one: the people here who know Japanese didn't say this trailer says anything about season 4. The second: when this trailer was released, it wasn't confirmed to be season 4.

Aside from that, I think it says something about a new anime project for High School DxD, or maybe specifically a new TV anime (project) for High School DxD.