I like the idea Cman gave, that the GB4 victims knew their killer as a repeat customer. I'll add that they don't just abandon their own customs to accomodate him on the nights of their abductions, but they travel from distances like CT and Maine to meet him. Is that done commonly?
Another thought about the remains: the remains of the mother and child form the outermost markers of the dumping ground, like parentheses. I think that's significant and purposeful on the killer's part. If MV is also LISK, it may send a territorial message. If not, the message may be competitive ('I'm bigger and badder than him,' say).
The GB4 are marked at 500 ft intervals (roughly a tenth of a mile apart, Costello a fifth of a mile west of the others), which may have a significance of its own. I've guessed before that it could indicate he's finding his way along the darkened stretch of road by reading his odometer - so, what about the distances between those and the additional remains? Or the placement of the additional remains vis a vis the distance to the road, versus that of the GB4 (roughly 50 ft north of the road, no?). I haven't looked at these things yet, but a match to that of the GB4 would suggest a link, where a mismatch suggests a different thinking. I suggest it because there's more than one criterion on which to base an unsteady conclusion (but hell, a reason to believe, if you're into that kind of thing). Maybe someone can suggest other logistical ways to measure the area, to narrow down what sub-conscious cues make the GB4 different from the others. Any number of killers is going to be thinking in terms of safety when bodies/remains are dumped.
I really like that odometer theory. From what I understand you can't necessarily freely walk through the brush where the four bodies were found because it's far too dense to do so. So that means he would have had to have judged the distance from inside his vehicle, moving slowly alongside the road to the desired dump site. I highly doubt he was carrying a burlap sack full of human remains in plain sight alongside a main road where he could easily be spotted. He picked his spot, jumped out of the vehicle, unloaded the sack, and took off. The whole process probably took 30 seconds or less if he hustled. Remember, the burlap sacks were found on top of the ground, and not even slightly buried, right? That tells me it was a run and dump type deal. He wasn't going for a scenario where there was no chance these bodies would ever be found, he just wanted them out of sight. Also, keep in mind that these kinds of guys get off on living right under your nose. They like the cat and mouse aspect of the hunt. What better way for these girls to get comfortable with him than for him to build a relationship of sorts with them? Something that took more than one encounter. I'd bet my life at least 2 of the 4 victims found in that area knew him as a repeat customer. And I'm sure whatever he offered to pay these girls was nothing to scoff at, either.