Mark Covell is in hospital in Genoa with serious injuries
The protesters said they had been wrongfully arrested and had endured four days of inhumane conditions before being released without charge on Wednesday.
Meanwhile, politicians in Genoa are calling for an investigation into the G8 policing operation, amid allegations the police sang Fascist songs and threatened protesters with rape and violence.
A police source has reportedly verified an attack by officers.
As Jonathan Blair, 38, Daniel McQuillan, 35, Richard Moth, 32 and Nicola Doherty, 27, were reunited with their families, they released a statement denouncing the police's conduct.
"Police indiscriminately batoned those present, mainly young people offering no resistance".
They described the place where they were detained as like a "field hospital in the Crimean War" where there were people with broken bones and head injuries.
The statement added: "We were held in bare concrete cells for 36 hours with little food and in conditions of severe mental and sometimes physical stress."
The Britons said requests to see lawyers were refused while passports and money were taken away.
Stephen Jakobi, of Fair Trials Abroad, said the delay in consular access had caused "misery, bewilderment and panic" among the protesters' friends and families.
The fifth protester, Mark Covell, 33, is still in hospital in Genoa with serious injuries.
He told the BBC he feared for his life during a brutal attack, which left him with a punctured lung, broken ribs and internal bleeding.
He said: "I ran smack into a Carabinieri. I didn't stand a chance".
Mr Covell, a journalist who works for a group that publicises anti-capitalist demonstrations, has pledged to begin legal action against the police.
He categorically denied he was involved in any street fighting.
But Italian authorities earlier defended the actions of the police, who, they said, were faced by a section of protesters intent on provocation.
The Italian Embassy told BBC News Online the cases were now being investigated by judges in Genoa, while Green Party Senator Francesco Martone has put down a question to be answered in the Senate in Rome on Thursday afternoon.
He told the BBC Radio 4's World at One programme that he had been given confidential documents suggesting the police were forewarned that Fascist groups would be taking part in the demonstration.
"We are going to be asking formally to the government to explain the behaviour and to have an investigation into the facts.
"This is a very big wound for the city. It's the kind of thing we used to experience 20 years ago."
One police source has reportedly compared the operation to a "nightmare under Pinochet's Chilean dictatorship" and accused some of his colleagues of behaving like fascists.
He told the respected Centre-Left daily La Repubblica: "They lined them up and banged their heads against the walls. They urinated on one person."
Genoa city councillor Giancarlo Bonifai told Today that he hoped the investigation into the allegations would be carried out as quickly as possible.
He said the city was still reeling from the weekend of violence in which one protester was shot dead by police.
And he said there would undoubtedly be a full inquiry and questions had already been asked in Parliament.