This is classic Y.A. fiction revolving around privileged teen-age girls. Petty rivalries and jealousies, pressure to excel (from parents), to fit in (from peers). Plus the added angst of having a secret that could destroy their lives and someone who knows the secret and threatens to tell. The four girls have no idea who knows and who they can trust, they don’t even trust each other.
“A” now shows he/she is not only a master manipulator that knows all their secrets, he/she will go to extreme levels to keep the four girls under her control. He/she will even kill to keep her identity hidden.