Purpose: To investigate dosimetric advantages of a 4D IMRT/VMAT planning method employing a segment aperture morphing (SAM) algorithm accounting for respiration-induced target motion and deformation. The minimal required number of calculated SAM phases was tested to minimize plan creation time. Methods: The new technique involves generating 4D step-and-shoot IMRT or VMAT plans based on 10-phase 4DCT in the following steps: (1) optimizing a flattening-filter-free IMRT/VMAT plan based on the 50% phase CT dataset (2) warping contours from the 50%-phase CT to the remaining phases via deformable image registration (DIR) and manual editing (3) morphing segment apertures of the 50%-phase plan to each phase CT using SAM that adjusts each aperture shape based on the change of the beam’s-eye-view of the target contour between the two phases (4) calculating the dose distribution for each phase with the morphed apertures and same MUs as in the 50%-phase plan, and (5) generating the 4D plan by accumulating the dose from all nine SAM-corrected phases and the original 50% optimized phase using DIR. 4D plans based on 10, 7, 5 and 3 SAM-corrected phases were generated for representative lung and pancreatic cancer cases, and were compared using DVH analysis against the gold-standard 4D plan where all 10 phases are individually planned and optimized. A SAM-corrected 4D plan was also compared to a simpler phase-by-phase isocenter-shifted plan to assess accounting for tumor deformation and translation opposed to only tumor translation. Results: Plan DVH quality for the 10-phase and 7-phase 4D SAM plans were comparable with the fully-optimized 4D plans (PTV-D95’s within 3% of fully-optimized plans). SAM based algorithms out-performed simpler isocenter-shifted-only approaches (∼1.5% PTV-D95 improvement). SAM-based 4D planning greatly reduces planning time verses individually optimized phase planning techniques. Conclusion: SAM with DIR provides a fast and accurate way to create 4D IMRT/VMAT plans.