![4girls :d >_< ^_^ a.i._channel bangs bare_shoulders black_dress blonde_hair blue_skirt blush boots breasts brown_hair butterfly_hair_ornament cleavage cleavage_cutout closed_eyes convenient_censoring crossover dennou_shoujo_youtuber_siro detached_sleeves dress eyebrows_visible_through_hair green_eyes grey_hair hair_ornament high_ponytail kaguya_luna kizuna_ai kurihara_sakura large_breasts long_hair long_sleeves mirai_akari mirai_akari_project multicolored_hair multiple_crossover multiple_girls open_mouth pink_hair pleated_skirt red_legwear shirt short_shorts shorts side_ponytail siro_(dennou_shoujo_youtuber_siro) skirt sleeveless sleeveless_dress sleeveless_shirt sleeves_past_wrists smile star streaked_hair the_moon_studio thigh_boots thighhighs twintails upper_teeth very_long_hair virtual_youtuber virtual_youtuber_shiten'nou wavy_mouth white_background white_footwear white_legwear white_shirt white_shorts white_sleeves rating:s score:4 post #3716773](/cdn_image/preview/63/c9/63c9af04ba8ac1495fe59874cfcbf082.jpg)
Used to refer to Kizuna AI, Kaguya Luna, Mirai Akari, and Mikoko ("Nojaloli"/Nekomasu) and Siro, in reference to the Shiten'nou (Four Heavenly Kings).
From the List Of Virtual Youtubers:
Referred to as the "Virtual YouTuber Four Heavenly Kings" online (バーチャルYouTuber四天王), and as "Virtual Oyabun and the Four Heavenly Kings" on Pixiv (バーチャル親分と四天王); other names include the Big Four, the "Council of Five Elders" (五大老) and the "Permanent Council" (常任理事国).
The first VTubers to get big, a group originally thought to consist of Kizuna AI, Kaguya Luna, Mirai Akari, and Mikoko. Due to the enormous subscriber gap between her and the rest of her peers, Kizuna ascended past the category (now nicknamed Oyabun, popularized by Luna), allowing Siro to join its ranks. These five would be considered the core, seen as the driving force behind the VTuber boom around the end of 2017 and start of 2018. The title was thus bestowed upon them not necessarily due to them being the Top 5 in subscriber numbers, but as a honorary title by fans recognizing their contributions.