As far as I know, Ginji is the Lightning Emperor (head of VOLTS) and the 4 kings of Infinity City serve him, but I knew only 3 - Shido, Kazuki and also Makubex.
This is actually something the manga tried to delibrately keep mysterious (it's one of the "misteries" listed on the GB official website). I'll look into it. I think I remember a picture somewhere.
Anisa.
Coordinator, Get Backers Manga Translation Project