Emergency crews were called to the Mernda train station in Melbourne's northeast on Friday evening following reports of a fight among teenagers.
A group of passers-by, including a 22-year-old Mernda man, stepped in to intervene after a group of teens set upon a 14-year-old boy.
The passers-by tried to diffuse the situation, with the man taking the boy and walking him away from the situation and outside the train station, Homicide Squad Detective Acting Inspector Nigel L'Estrange said.
"Unfortunately, those four offenders followed both of them and then, in what could only be described as a savage attack, assaulted the 22-year-old male physically, both fists, kicking, and they used edged weapons," he told reporters on Saturday.
The victim suffered life-threatening injuries and was treated by paramedics but died at the scene.
Four male teens - a 16-year-old, two 17-year-olds and an 18-year-old - were arrested.
The 16-year-old, from Whittlesea, and a 17-year-old from Geelong as well as an 18-year-old from Mill Park were all charged with murder and armed robbery.
The eldest suspect will face Melbourne Magistrates' Court on Sunday.
A 17-year-old from the City of Yarra is under police guard at hospital and yet to be interviewed.
"This is just a tragic situation where our victim here is just a good Samaritan who was trying to do the right thing," Det Insp L'Estrange said.
"He's intervened, tried to protect this 14-year-old schoolboy, hasn't been aggressive at all, tried to remove him from the scene, and as a result of his interaction, he's now lost his life."
Health and Ambulance Services Minister Mary-Anne Thomas said her thoughts were with the victim's family, calling the horrific incident really concerning.
She referred to the government's toughened youth crime laws, which came into effect just over a week ago and could result in youth offenders being dealt harsher punishments for serious crimes.
"Young people that are committing violent crimes are about to discover that they will be doing adult time," Ms Thomas told reporters on Saturday.
Opposition police spokesman Brad Battin said the incident highlighted Labor's failure to tackle youth crime and remove machetes from the streets.
"With nearly 2000 vacancies on police rosters, more than 40 stations closed or operating on reduced hours and PSOs to be stripped from 120 train stations, Victorians continue to pay the price for Labor's community safety failures," he said in a statement.
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