Assuming this refers to four of the Lincoln Assassination conspirators, the execution took place in a courtyard of the Washington Penitentiary, which had been repurposed as the Washington Arsenal in 1862. One building survives - that in which the trial of the conspirators was held - located in Fort McNair, Greenleaf Point.
On the off chance.... four men were hanged August 22, 1864 for conspiracy in the town of Quitman, GA. John Vickery (a white man) and three slaves named Nelson, George and Sam planned to take over an arsenal and foment a slave insurrection.