A fifth man has been arrested after an undercover documentary exposed the alleged abuse of children at a youth custody centre.
The BBC Panorama programme was filmed at Medway Secure Training Centre in Rochester following reports from a whistleblower.
All five men - who were held on suspicion of child neglect - have been released on bail until April, pending further investigation.
Three men from Medway, aged 25, 34 and 35, and a 25-year-old man from East Sussex, were arrested on suspicion of child neglect.
A fifth man, aged 28, was arrested on suspicion of assault.
The footage, shot by an undercover journalist who posed as a security officer, appeared to show staff using excessive force to restrain youngsters, bullying, lying when reporting incidents and boasting about hurting the inmates.
When the allegations emerged, seven staff members were suspended. In a statement yesterday, G4S said four of these have now been sacked.
The other three remain suspended while investigations continue. An additional member of staff has been removed from operational duty.
The BBC Panorama programme was filmed between October and December last year.
Footage appears to show a senior officer restraining a 14-year-old boy by pressing his fingers on his throat.
In another scene, a vulnerable boy who was having his room cleared of anything he could harm himself with, was apparently choked and slammed on a bed.
The programme also revealed officers lying about incidents to cover up their actions.
G4S said it has requested copies of all of the evidence collected by the BBC in order to carry out its own investigation.
“We are appalled by the behaviour of certain members of staff at Medway Secure Training Centre shown in the programme and I would like to apologise personally to any young people involved in these incidents" - Paul Cook
Commenting after the broadcast, Paul Cook, G4S managing director of G4S children’s services said: “We are appalled by the behaviour of certain members of staff at Medway Secure Training Centre shown in the programme and I would like to apologise personally to any young people involved in these incidents.
“There is no place for the conduct shown in the programme within any of our Secure Training Centres and it will not be tolerated.
"We took immediate action to suspend the staff concerned on 30 December and we will take further action as appropriate once the police investigation is complete.
"We have also taken immediate steps to increase the oversight and governance at Medway whilst the investigation continues.
“We are in close discussion with the Youth Justice Board to commission an investigation into these matters once the Kent Police investigation is complete.”
Earlier this week, shadow justice minister Andy Slaughter said all G4S run prisons and centres should be placed into special measures following the investigation.
The centre, run by private security firm G4S since it opened in 1998, looks after 76 boys and girls aged 12 to 17.
The chief executive of The Howard League for Penal Reform said the programmae made her cry.
The charity, which offers support and advice to young people in custody, said its legal team has dealt with numerous concerns raised by or on behalf of young people at Medway dating from at least 2008.
The team has also worked with adults who were detained there as children and who have raised concerns about their treatment.
Frances Crook, chief executive, said: “Watching this programme made me cry. The deliberate cruelty against children was one of the most upsetting things I have seen in this country. Shocking also was the institutionalised fraud being perpetrated to cover up that abuse.
“The children in Medway must be found other places within the next few days because this institution is rotten to the core. The contract should then be rescinded.
“The government ought to explore whether G4S should repay the taxpayers’ money it has received in the last few years. It has been paid to look after children and it has failed.”