In MR-guided abdominothoracic radiotherapy, fast 2D imaging for MLC tracking and respiratory-correlated 4D imaging for dose accumulation is desirable. We developed a hybrid 2D/4D MRI method for pre-beam and beam-on imaging, which continuously acquires and reconstructs T2w coronal simultaneous multi-slice (SMS) images. Pre-beam 4D-MRIs were used to calculate a 3D mid-position reference image. Beam-on SMS data were used to extract real-time motion using a 4D-based motion model, and continuous 4D-MRIs. Dynamic updating of the 4D-motion model yielded the best real-time motion estimation. The presented hybrid 2D/4D MRI method can potentially facilitate novel MR-guided abdominothoracic radiotherapy workflows.