A method for determining a 4D plan for carrying out intensity-modulated radiation therapy of a target volume subject to irregular periodic motion, with a radiation therapy apparatus is provided. The method includes selecting positions of a radiation source. The number of positions is selected to be identical in all 3D radiation therapy plans. The method also includes selecting a number of respective aperture settings assigned to a respective position of the radiation source to be identical in all 3D radiation therapy plans. A geometrical, temporal, and/or dynamic restriction that restricts a change of the aperture from one aperture setting to another aperture setting is predetermined, and the 3D radiation therapy plans are determined such that the 3D radiation therapy plans fulfill predetermined restrictions for aperture settings in each case.