A special page on Natsume Yuujinchou Roku's official website has announced that the series will receive two new unaired episodes. Titled Suzunaru no Kirikabu and Mugen no Kakera, the OVAs will be bundled with the fourth and fifth Blu-ray and DVD volume on September 27 and October 25, respectively.
Drawn by Yuki Midorikawa, Natsume Yuujinchou began its publication via Hakusensha's LaLa DX magazine in 2003, and moved to LaLa magazine in 2008. The series currently has 22 compiled volumes in print, with the latest released on September 5. Brain's Base adapted the manga into a TV anime in Summer 2008 and produced three more seasons before studio Shuka took over the production in Fall 2016 for the fifth season. The sixth and most recent season aired in Spring 2017.
PV for Suzunaru no Kirikabu
Note: The video is region-locked; click here for an alternative link.