PET/MRI is capable of simultaneous respiratory motion-resolved four-dimensional (4D) PET/4D MRI data acquisitions. Therefore, it is important to develop a clinically applicable method for respiratory phase-matched MR-based attenuation correction (MRAC) for accurate 4D PET quantification. This study proposed 4D MRAC protocols modifying the current MRAC protocol and evaluated their performance using a patient with liver metastases. For qualitative analysis, 4D MRAC improved phase-mismatch at the lung/liver interface substantially. For quantitative analysis, 4D MRAC improved PET quantification by 10-30% increase of PET-avid tumor uptake values, compared to static MRAC, specifically for the tumors located at the lung/liver interface.