I just saw that it's still an ongoing manga, so I was wondering what chapter was the last serialized. Also, what other chapters of the manga are serialized and what's left out?
According to MangaUpdates:
Starts at Volume 1, Chapter 1 (Season 1)
Ends at Volume 6, Chapter 118 (Season 3)
These is a season 4 on top of that, but I couldn't find what it adapts.
Apparently the second season deviates from the manga or more accurately, includes non-manga episodes/content.
An excerpt from the link above:
It is also different in storytelling as it uses material not from the manga
As of August 2014, 12 tankōbon volumes of the series have been released, which as you say is still ongoing.