[Freesurfer] convert point from anatomical- to functional space

Dear Freesurfers,

I have a question concerning the register.dat. In Freesurfer-Wiki I found that sentence: 'The last four lines of register.dat have the values of the registration matrix that converts a location in anatomical space to a location in functional space' I want convert one special point from my high resolution anatomical space (256x256) to my low resolution functional space (64x64). So I want apply the register.dat-Matrix to the Vektor from this point in anatomical space. But why is it an 4x4 Matrix? I supposed that I need an 3x3-matrix for an translation in x,y,z-direction. How can I get this anatomical point in my functional space? Thanks for your answer