User blog comment:HollowIchigo58/High School DxD Season 4 Announced/@comment-2143738-20161022210715/@comment-9595671-20161025051226

I think DxD is one of the longest runners, I only know of one other Light novel anime that's managed to reach four seasons and that's Familiar of Zero, and I feel it was partially to help give it an ending cause of it's author's death before it was revealed it could still be finished.

So I'd say DxD has a good chance to break the mold and reach a 5th season. Sword Art Online could possible do this, but the current arc despite ending is 10 volumes long... not exactly something you can fit into a third 24-26 episode season.