Board Thread:Questions and Answers/@comment-26359879-20150508000003/@comment-26215551-20150508004309

If I remember correctly, Kogitsunemaru isn't a national treasure (he comes from a mythical tale I believe? Someone correct me if I'm wrong). So that's why he doesn't get a Rarity-4 rating like the other national treasures, such as Hotarumaru and Kousetsu.

Though it seems that national treasures of other sword types (e.g. Atsu and Hirano are Tantous, and Heshikiri is a Uchigatana) keep their normal rating and just have slightly longer smithing times...