This tweet refers to "the 4 people of Back Gas Mask" so it seems to be a unit:
https://twitter.com/staff_sayuri/sta...87979810689024
From this Facebook post we can see Haruka, Tetsuyuki Yamada, Yo Fujii, and Takehiro Kojima:
https://www.facebook.com/tetsuyuki.y...04350862949035