1. A method for distinguishing between four materials (M 1 , M 2 , M 3 , M 4 ) in tomographic records of a dual-energy Computed Tomography (CT) system, the method comprising: scanning of an examination area of an examination object, including at least four different materials (M 1 , M 2 , M 3 , M 4 ), wherein the scanning is carried out using two different X-ray energy spectra from a multiplicity of projection directions, and absorption data is measured for each X-ray spectrum that is used, Hounsfield Units (HU) values of four materials (M 1 , M 2 , M 3 , M 4 ) form a quadrilateral on an HU-value diagram of the X-ray spectra used for scanning, the two diagonals (M 1 -M 4 , M 2 -M 3 ) of which quadrilateral intersect one another, the materials which form one diagonal are each mixed, and two materials which define a diagonal form a mixture with particle sizes in the same order of magnitude as the resolution of the CT system; reconstructing at least one first tomographic image data record from the absorption data of the first X-ray spectrum; reconstructing at least one second tomographic image data record from the absorption data of the second X-ray spectrum; defining a size of at least one of a two-dimensional and three-dimensional viewing area around each voxel whose material content is to be distinguished, and carrying out subsequent statistical calculations in the viewing area; imaging adjacent voxels from the viewing area on an HU value diagram, for each voxel whose material content is to be distinguished; calculating distances (x 1 , x 2 ) from the diagonals (M 1 -M 4 , M 2 -M 3 ) for all the imaged voxels, and forming a mean squares of the distances ( x 1 2 , x 2 2 ); and assuming, when the mean square distance ( x 1 2 , x 2 2 ) to one diagonal is less than to the other diagonal, that the composition of the voxel is composed of the materials (M 1 +M 4 , M 2 +M 3 ) to whose diagonal the lower mean square distance ( x 1 2 , x 2 2 ) occurs.