I believe that whoever killed the AC4 was from that immediate area. He had to know that seedy part of town and been a part of it. Perhaps a drug dealer? I used to drive by the Golden Key on my way out of town just about every weekend in 2006. "Normal" people knew that was NOT a place you wanted to be. It was downright scary and I am not the type that scares easily either. (I even met Denise Hill on my way out of town one early Monday morning when I stopped to get gas next to the GK.) That was not just a place he picked to turn into his dumping ground. He knew the place and most likely stayed there. I believe they were all killed there in the motel. That makes the most sense, in room 101 to be exact. There was direct access to the ditch from that room. If you have seen pics of the Golden Key, it is (was) a horseshoe shaped building. If someone came there to dump bodies, they would have to risk parking in the lot and carrying the body around the building. People came and went from there at all hours from dusk til dawn and it would have been a big risk of getting caught.
As far as the AC4 and the GB4 being related, I think it is very possible that he moved on to LI after the bodies were discovered in AC. If he did, he also "evolved" from street walkers to escorts on the internet. That was more out of necessity. I don't think Long Island was known for it's street walkers. Because he used the internet, he was able to choose the type of girl he liked (young, of small stature, etc.) He probably got to know these women through talking to them, and possibly saw them more than one time, making them let their guard down easier. I think this because of the way that some of the girls left their things and didn't follow their "normal protocols". I also think that there will be more discoveries of bodies in the future. They will be scattered though due to Hurricane Sandy..