First, I'm so sorry for your loss. I know too well the sadness of losing a mother to this dreadful disease.
Grief is very personal and everyone deals with it differently. It doesn't have a specific time frame. While four years might seem like a long time to some, it can take much longer than that for others to heal. Give yourself permission to feel the way you do. The fact that you recognize your anger is a big step.
Can you pinpoint what specifically makes you angry? Are you angry at yourself? At your mother? Is it the reminder of what she went through? Do you feel her doctors or you could have done more for her? Are you angry that your time with her was cut short? There's always so much more we wish we could do or say to a loved one after they've passed. Are you angry at choices she made in her treatment? Do you think she could have fought harder? Perhaps you're angry at knowing that cancer can strike anyone, and you could be next? Could you be angry at survivors because they survive, or at organizations who raise funds and their effective use of those funds?
Maybe if you recognize your mother's personal fight and honor her bravery your anger will ease.
Cancer brings out a lot in people, especially survivors and those who care for them. Some might still be in treatment while some finished treatments decades ago. Their purpose in raising awareness is to help others, like yourself, become informed about the disease, its symptoms, treatments and what you might do to prevent it. They celebrate life and the bond between people touched by breast cancer.
You might start by joining a local breast cancer support group. There are groups for survivors and for caregivers. You are now part of a huge community who welcome and understand you and the emotions you are wrestling with. Sharing your story with others can be an immensely healing experience for you and for those who hear it.
I wish you peace.