Tappei released the first three light novels monthly back to back. That's very fast release times for a light novel and was mostly due to the web novel source already being written.
Once they got to the third arc writing became more complicated due to plans for an anime production. The author took a very active part in the animes production, due to this the light novel releases slowed significantly.
With the release of the 9th volume in a few days (September 25, 2016) and the end of the anime production (September 18, 2016) both of those problems are gone. Volume 10 is set to release next month as well so we're back to monthly and bimonthly releases.
The 4th arc should translate into about 9 to 10 novels, it should take a year to a year and a half at most for Tappei to finish the 4th arc. That's enough content for another 2 cour season on it's own. So I'd put the second season spring or summer of 2018.