Four masked muggers wearing all black stabbed a 15-year-old boy they didn’t know in a robbery in Hell’s Kitchen during Monday’s evening rush, cops and law-enforcement sources said. The callous crew approached the teen around 5:20 p.m. on West 51st Street near 10th Avenue in Manhattan and knifed him twice in the lower back during the vicious crime, police said.  The thugs grabbed the victim’s cell phone and backpack and fled, authorities said. The teen victim was rushed by EMS to Weill Cornell