WARNING: This page is about a different car, the 2025 BMW M4, 2024 BMW M4, 2023 BMW M4, 2022 BMW M4, and 2021 BMW M4. However, it is still accessible from the selected car via links, so may be relevant.
Unclip the rear seat upwards out of the clamps (1) and feed out the seat belt buckles (2).