The killers behind four execution-style murders in Melbourne, including the death of horse trainer Les Samba, are unlikely to ever be known, police concede.

State coroner Sara Hinchey on Thursday ruled the murders of Mr Samba, brothers Vincenzo and Gerardo Mannella and businessman John Furlan would remain unsolved unless new evidence came to light.

The Coroners Court heard the deaths remained open police investigations, but Judge Hinchey found that any probe she undertook was unlikely to yield the killers' identities.

The anti-gangland Purana taskforce investigated Mr Samba's 2011 death and the Mannella murders in 1999, but no charges were ever laid. The arson and explosives squad investigated Mr Furlan's death.