Alison's death loomed large over the fourth season, as flash-forwards revealed that the grief counselor would meet her demise sooner rather than later. Before we get to that point, however, the season puts her through the wringer, with her falling for Ben, a man with some serious, possibly violent trauma from going to war and from alcoholism; she also discovers that she is the product of rape.
The subject of her death is a complicated one, since we're shown two different narratives and not told which one is "true." In one, it's implied she took her own life; in the other, Ben is a much darker and violent character and murders her. By the time the season ends, we still aren't quite sure which version of events — if either — is the truth behind her death.