Four people are in custody after a possible smoke bomb went off during an anti-Muslim protest outside Gracie Mansion, causing a mass panic on Saturday. Police said there were no injuries and no explosion at the scene, where dozens of demonstrators and counter-protesters hastily fled after the smoke started to billow from a device that was thrown by an individual at the crowd. “It looks like a possible smoke bomb. We’re not tracking any injuries,” police sources told The Post. One person is in