Menendez had earlier read a prepared statement in which he justified the military's actions as necessary in the face of a threat by left-wing activists.
"We had to take appropriate measures," he said.
The verdict is seen as significant because of Menendez's gruesome reputation and because he was ordered to serve the rest of his sentence in prison.
Many convicted former military officers are serving their sentences under house arrest, under an Argentinian law that applies to those over 70 or in poor health.
Six other former military officers and one civilian were also convicted on Thursday and given sentences ranging from 18 years to life.
The four victims, Hilda Palacios, Humberto Brandalisis, Carlos Lajas and Raul Cardozo, were kidnapped in 1977.
According to prosecutors, they were taken to the prison and torture centre known as La Perla, which was run by the Third Army Corps, commanded by Menendez, and killed the next month.
Their bodies were then dropped in the street to make it seem as if they had been killed in a gun battle.
Palacios was found in 2004 in a local cemetery. The bodies of the others have not been found.