Class-IV: Class IV has a single member, HDAC11, whose structure bears some resemblance to Classes I and II HDACs, but is not similar enough to be placed in either class. The specific roles of HDAC11 are still being elucidated, but it is notable that its structure is evolutionarily conserved, not only in vertebrates and invertebrates, but also in plants. (1)

Reference
1. Koeneke, E., Witt, O., & Oehme, I.HDAC Family Members Intertwined in the Regulation of Autophagy: A Druggable Vulnerability in Aggressive Tumor Entities. Cells.2015;4(2):135-168. PMID: 25915736.