The president was assassinated at his home near the capital, Port-au-Prince, and suspicion is directed at a group of mercenaries, most of whom came from Colombia.
The trial of the four began two months ago; the charges included planning to assassinate or kidnap the president, and materially assisting the plot.
According to prosecutors, the group was active in Florida and had decided who would replace the president if he were eliminated.
All were found guilty of crimes that could carry life imprisonment in the United States. Sentences will be announced later.
Several people have previously been imprisoned or convicted for involvement in the president's assassination.
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