I find this article very interesting:
http://people.com/crime/long-island-serial-killer-gilgo-beach-fbi-profilers-experts/?sf40641419=1
One point mentioned, because they go back and forth as to whether there is one serial killer or two, is that there's one killer that has changed hi MO over the years because he didn't feel safe anymore. Then why keep the same dump site I say?
If you change your MO because you don't feel safe you most likely change your dumping ground too. I believe there are definitely two different killers that just happen to be dumping at the same place. The one from Mannorville either got arrested already for something else or has died. But the GB4 killer is still on the loose and has already mapped out a new area. He just doesn't want to get caught. So, he's waiting things out a bit.
The FBI and police may have even interviewed him and this may have slowed him down as well knowing that their eyes are on him.
Some of the best clues would be old Craig's List records of the conversations I believe.