well im one that has always advocated finding another way to deal with these kinda thing and would hate to shoot a dog. BUUUTTTT at this point things may be progressing past the point of that. Once they started forming a pack like what your talking about and showed even the slightest sign of aggression that would be it, its not worth taking the chance on your own well being, one dog is no big deal but four would be hard for any man to deal with. If they are also showing pack behavior they may just be out right strays, best advice here would be if they show up and do it again to try and determine which is the alpha and put him down, hopefully that would get the message to the rest of the pack