MIAMI — More than a year and a half after launching their investigation, U.S. federal agents arrested four suspects in South Florida Tuesday on charges of playing key roles in a plot to kill Haiti’s president, Jovenel Moïse, who was shot to death in his home by an alleged team of Colombian commandos as his presidential guards stood down. The local arrests of the owner of a Miami-area security firm, his business partner, a financier and an exporter mark a turning point in the probe that is now focused on the weapons, ballistic vests and financing that authorities say fueled the deadly plot exec...