長船四天王 Osafune Shitenno

What is Osafune Shitenno in Japanese Katana terminology?

"Osafune Shitenno" refers to the four swordsmiths who were representative of the group of swordsmiths in the Osafune region of Bizen Province (currently Okayama Prefecture). These names were proposed by Hon'ami Kōetsu, a sword appraiser from the Edo period. The four swordsmiths are "Nagamitsu", "Kanemitsu", "Motoshige", and "Nagayoshi".