Bah, schutzhund and GSD's are for sissies...get yourself a Mal! Here are some real dogs:
http://www.youtube.com/watch?v=hc5P8n3LRak
I've got two Belgian Malinois in my yard, one a 2 1/2 year old female dope dog and the other a 5 year old male who has worked for two departments and had 6 good bites on bad guys (before I got him). I'd have loved for those 4 guys to have come into my yard. I even have the sign up in my window, "Belgian Malinois: I make it to the fence in 2.8 seconds, can you?"
I'm sorry for those here who feel that these are just kids fooling around. 12 or 13...sure, maybe. The OP said they were 18 to 20. I'm sorry but if four 18 to 20-years-olds came BACK into my yard they'd deserve just about anything they had coming to them. Good job to the OP! And trespassing (misdemeanor in my state) definitely equals a detention. Hold them at gunpoint until a cop gets there? Sure. If I ever got called to a scene where a homeowner held 4 twenty year olds at gunpoint for breaking into their yard I'd give the homeowner a high five.