The four present in court were identified as Cisse Hantao Ag Mohamed, Kounta Sidi Mohamed, Cisse Mohamed and Barry Hassan.
The others, including the suspected masterminds, are either on the run or being held in Mali, said Aude Rimailho, a lawyer for civilian plaintiffs.
"The people who planned the operation are in Mali," she said.
The charges against the 18 include acts of terrorism, murder, attempted murder, criminal concealment, illegal possession of firearms and ammunition "and complicity in these deeds," Public Prosecutor Richard Adou said last week.
Nineteen people were killed -- nine Ivorians, four French citizens, a Lebanese, a German, a Macedonian, a Malian, a Nigerian and a person who could not be identified.
Thirty-three people of various nationalities were wounded.
The four present in court were identified as Cisse Hantao Ag Mohamed, Kounta Sidi Mohamed, Cisse Mohamed and Barry Hassan.
Rimailho, representing French plaintiffs, said in the runup to the trial that the 18 were "small fry" and cautioned against seeing the proceedings as a chance for closure.
The attack on Grand-Bassam was the first and deadliest in a string of sporadic attacks on countries lying on the coast of the Gulf of Guinea, south of the Sahel.
In Ivory Coast's case, the assault had a deeply chilling impact on foreign tourism, an important money-spinner in an economy battered by a post-election conflict in 2010-11 that claimed 3,000 lives.
In January 2017, members of France's Barkhane anti-jihadist force captured a key suspect, Mimi Ould Baba Ould Cheikh.
He is described by Ivory Coast investigators as one of the instigators of the Grand-Bassam attack and by Burkina Faso as the "operation leader" in an assault on the Burkinabe capital Ouagadougou in January 2016 that claimed 30 lives.