Apparently 19 Families have Heads of Family
Nine of them are Holy Knights, Five are Gorosei, and another One is Imu
This leaves room for Four Others
This could be the Secret of WG's Flag
Remaining Four alongside Imu represent those Five Points on Flag (Imu is Middle One)
Similar to how Revos have Commanders of Four Blues + Grand Line (With Ivankov being Okama Queen)
WG have Imu as King/Queen of Grand Line & there are Four others for Four Blues
However, just like Most Villains, i believe Imu is also a Traitor & He betrayed Four Others
There is High Chance, They are Crocodile, Mihawk, Moria & Kuma
(Imu made sure They lose their Claim for Empty Throne, basically similar to how Orochi's Grandfather wanted to Rule Wano)