Milon: A demon of Malebolge
Kainazzo: Sea of Mystery (or something like that)
Valvalis: I have absolutely no idea...
Rubicant: Ruby---? (or something like a demon of canto 22)


*If* you meant the original japanese names, they are the following:
Milon -> スカルミリョーネ (Scarmilione)
Kainazzo/Cagnazzo -> カイナッツォ (Kainazzo)
Valvalis -> バルバリシャ (Valvalisha)
Rubicant -> ルビカンテ (Rubicante)