The violence that erupted over the July Fourth weekend left four people dead and 15 others wounded in shootings and stabbings across Boston, alarming police officials who said they are stepping up patrols in crime-plagued areas to prevent retaliatory violence.

Most of the violence came in just five hours on one of the city’s bloodiest Independence Days in years. Four men - including a 17-year-old Roxbury high school senior who died in his father’s arms - were killed, and nine others were wounded in that period between Monday night and yesterday morning. All four killings were connected to gang feuds, said Police Commissioner Edward F. Davis.