They were working beneath the M5 at Avonmouth Bridge, near Bristol, when the platform on which they were standing gave way.
The access gantry collapsed and fell 80 feet into a car compound.
£100m contract
The accident happened shortly before midday on the northern side of the River Avon. No one else was hurt.
The bridge repair works are part of a five-year £100m project.
Two main contractors, Costain and Kvaerner are carrying out the work for the Highways Agency. The dead men were working for Kvaerner.
The Health and Safety Executive (HSE) has already launched an investigation and the emergency services are still at the scene.
A Highways Agency spokeswoman said: "The incident is being investigated by the Health and Safety Executive and the police and fire services are both involved.
"The bridge is still open to traffic."
Specialist steel workers
A Kvaerner spokeswoman said three of the dead were employed by a Darlington-based subsidiary called Cleveland Bridge and the fourth was working for another subsidiary called Motherwell Bridge, which is based in Scotland.
Two of the men were welders and two were specialist steel platers, but it is not known where they lived.
A spokesman for Avon and Somerset Police said parts of the gantry fell into the river Avon and although most was left hanging under the bridge.
He said: "A large part of the gantry, about 35ft long and 12ft wide, ended up dangling underneath the bridge."
Workers on a nearby industrial estate were evacuated after gas bottles fell off the gantry.
Several eyewitnesses were treated for shock.
Police said a man aged 42 from Newport, whose name has not yet been released, was among the four victims.
Police say people worried about their relatives being involved can ring a special telephone hotline, 0117 945 5599.
Northern Ireland
Scotland
Wales
England
The BBC is not responsible for the content of external internet sites.
Next steps for peace
Blairs' surprise over baby
Bowled over by Lord's
Beef row 'compromise' under fire
Hamilton 'would sell mother'
Industry misses new trains target
Quins fightback shocks Cardiff
(From Sport)
Vodafone takeover battle heats up
(From Business)
IRA ceasefire challenge rejected
Thousands celebrate Asian culture
Christie could get two-year ban
(From Sport)
Colleagues remember Compo
(From Entertainment)
Mother pleads for baby's return
Toys withdrawn in E.coli health scare
Nurses role set to expand
(From Health)
Israeli PM's plane in accident
More lottery cash for grassroots
Pro-lifers plan shock launch
Double killer gets life
Cold 'cure' comes one step closer
(From Health)
Straw on trial over jury reform
(From UK Politics)
Tatchell calls for rights probe into Mugabe
Ex-spy stays out in the cold
Blair warns Livingstone
(From UK Politics)
Smear equipment `misses cancers'
(From Health)
Boyzone star gets in Christmas spirit
(From Entertainment)
Fake bubbly warning
Murder jury hears dead girl's diary
Germ warfare fiasco revealed
(From UK Politics)
Blair babe triggers tabloid frenzy
Tourists shot by mistake
A new look for News Online