1. A method for four-dimensional (4D) intensity modulated radiation therapy (IMRT) planning for motion compensated treatments, comprising: creating a deliverable IMRT treatment plan for a tumor in irregular periodic motion, said irregular period of motion comprising a plurality of phases, said treatment plan comprising treatment plans for each of said plurality of phases, each said treatment plan being delivered using a multi-leaf collimator, leaf positions of said multi-leaf collimator being adapted separately for each respective phase and for each of a plurality of beam positions, the beam being collimated by said multi-leaf collimator.