The accused say they acted under orders to kill Firmin Mahe, who was suffocated with a plastic bag while travelling in a French military vehicle.
French peacekeepers were deployed after a failed coup attempt in 2002 which split the country into the rebel-held north and the government-controlled south.
Mahe's death on May 13, 2005, led to the sacking of General Henri Poncet, who was in command of the 4,000-strong peacekeeping mission.
Poncet was relieved of his functions and given an official reprimand following a suspected cover-up of the murder.
The French military says Mahe was a murderer and rapist who terrorised people in the UN-occupied safe zone.
But his family says Mahe was not a criminal and argue that the wrong person was targeted.
They were not present at the trial, because their visas had not arrived, a fact which their lawyer suggested reflected a lack of political will from the authorities.
Mahe was taken in for questioning near the western town of Bangolo after being wounded in the leg during a clash with French military.
He was taken to an infirmary, then ordered driven to the city of Man by General Poncet, but died en route.
The four accused include the driver, 32-year-old ex-corporal Lianrifou Ben Youssouf, and 35-year-old ex-corporal Johannes Schnier who allegedly helped former sergeant-major Guy Raugel, 48, to suffocate Mahe.
Raugel says he acted under orders from co-defendant Eric Burgaud, a 50-year-old former colonel.
Burgaud accused Poncet, who at the time oversaw the 4,000 French troops stationed in the Ivory Coast, of ordering the killing with the phrase, "drive slowly, you understand me?"
"I understood the same as everyone else, that is, that it would have been best if Mahe arrived dead." Burgaud told investigators.
Poncet denies the claim.
Starting on Tuesday, the defence is expected to show the difficult circumstances that troops faced to maintain order without a legal framework.
"There is a political mandate, but in practice it's 'sort it out yourself'," said Alexis Gublin, Burgaud's lawyer.
The trial will continue until December 7.
The facts of the death came out at a tense time, six months after nine French soldiers died in a camp bombing near the central city of Bouake. Paris retaliated by destroying the Ivorian air force.