My guess is the four would be TT, DA, CA, and Melvin. It's possible that they haven't cleared VB, but I don't consider him a viable suspect. Billy Simpson is a pretty creepy guy, but there doesn't seem to be anything connecting him to the crime, and he can't be described as someone the girls trusted. The late-night sighting with Danny Wilson doesn't make a lot of sense; the girls had already been missing for hours.
Melvin was the creep who had originally followed DA and called to the Arnold House looking for Rachel on another occasion ?
For him to be the abductor would it not also mean that Rachel was the author of the 'Runaway' letter ? Not ruling him out, bit I think that has to be factored in.