This study is for people with a specific kind of breast cancer: Estrogen Receptor-Positive (ER+), Human Epidermal Growth Factor Receptor-Negative (HER2-). The goal is to see if a new treatment (GDC-4198) alone or with another drug (giredestrant) works better than a current treatment combo (abemaciclib and giredestrant). The study has two main parts: Phase Ib checks safety and how your body handles the drug, and Phase II compares how well the new and old treatments work. Participants need to have breast cancer that is advanced or has spread and must have tried certain treatments before. However, they can't join if their cancer is too aggressive or if they've had certain treatments recently.

• Study Duration: The study has two phases, but exact length isn't specified.
• Visits Needed: Participants will need to attend regular visits for monitoring.
• Potential Risks: There may be side effects from the drugs, and certain health conditions may prevent participation.