"This is clearly a lost opportunity to restore justice in Angola, and particularly in Cabinda," Lisa Rimli, a researcher for the Africa division at New York-based Human Rights Watch, said.
The Front for the Liberation of the Enclave of Cabinda (Flec) has claimed responsibility for the January attack in which two members of the Togolese football delegation were killed as the team was travelling by bus to the Africa Cup of Nations.
The four men convicted on Tuesday were arrested shortly after the attack.
The trial has been sharply criticised by rights groups that have accused the government of using the attacks to justify a crackdown on critics.
Martinho Nombo, a lawyer and rights activist taking part in the court hearings, said the judge convicted them because they had spoken or written about independence for Cabinda.
"This is unconstitutional.
A judge cannot jail someone for nothing"
lawyer and rights activist
"This is unconstitutional. A judge cannot jail someone for nothing," he said.
"This will only worsen Angola's poor record on human rights and the whole peace process with Flec."
The AFP news agency said the men had been arrested because they had documents about Flec and had travelled to Paris for meetings with exiled leaders.
"The supposed link was implied rather than stated," Rimli said. "They were convicted on the basis of those documents."
A total of nine people were arrested in connection with the Togo attack, but only two of them are suspected of having any direct link to the shooting, according to Human Rights Watch.
Radio Ecclesia said demonstrators gathered outside the courthouse on Tuesday to protest against the trial, as heavily armed riot police guarded the building.
Flec has for more than three decades been fighting for the independence of Cabinda, an oil-rich province that is separated from the rest of Angola by a strip of territory belonging to the Democratic Republic of the Congo.
The group says the Cabindans get too little of the oil revenues coming from their homeland.