Being honest about who you are with your friends is really important. It helps them trust you, and helps you trust them also. And there are all kinds of secrets. It sounds to me like you didn't going into this friendship with the idea of keeping secrets in mind, or of deceiving her from the outset. Four years ago you probably didn't know that you were bi, and it probably wasn't much of an issue in your friendship.
People change and grow and realize things about themselves all the time.
So the fact that you now know that you are also bi, doesn't necessarily mean that you have kept a deceptive secret from her. You're right that she might have reason to feel like you betrayed her trust, or like you have kept her from knowing something relevant to your relationship. But the good news is that the sooner you tell her, the less harmful this secret will be to your relationship. Now that you know you are bi, the longer you wait to tell her, the more of a secret it will be, and the more she will feel betrayed when she finally finds out. The more betrayed she feels, the less likely it is that you will be able to continue having such a close friendship.