When Rev Parris leaves, what does Abigail, Mercy, Mary & Betty's discussion reveal and how does it hint at events that will occur later?
After Parris leaves, Abigail tells Betty that she's told her father everything, and so there's really no reason to be afraid. It's a lie. Abigail's casual willingness to lie to her cousin (and to physically strike her) shows us her character and how capable she is of dissembling as well as harming others. She thinks nothing of it.
Moreover, learning that Abigail drank a charm to kill Goody Proctor hints at her absolutely willingness to do anything, however sinful or wrong, to get rid of this woman. We cannot be surprised later when we learn that she has accused her former lover's wife of witchcraft, and we must assume, with Elizabeth, that she does so in order to eliminate her and take her place.
Further, Mary Warren's desire that the girls should tell the truth about their activities in the forest hint at the events that take place in Act III. Proctor brings her to the court to confess that she and the other girls have been lying. Then, as now, Abigail's threats (veiled or quite open) succeed in compelling Mary to keep quiet and support Abigail's claims.
The discussion between the girls reveals what has actually occurred in the woods. The girls were dancing around a fire. It also comes out that Abigail drank blood in order to cast a spell to kill Elizabeth Proctor. When Mary Warren pleads with the girls to confess to dancing, Abigail's ruthless nature is revealed. She threatens the girls, telling them she will come in the night and kill them if they tell of what happened. Abigail is clearly in control of the girls, and they fear her vengeance. This discussion leads us to believe that later on the girls will be too afraid of Abigail to change their stories. It also reveals that because of her ruthlessness, Abigail will eventually succeed in her original mission, to kill Elizabeth Proctor. It reveals that Abigail’s manipulation of others and powerful presence will eventually persuade the courts to take her side.