The magnetization of single crystal Gd5Ge4, which in a zero magnetic field orders antiferromagnetically at 128 K, indicates a reversible spin-flop transition when the magnetic field is along the c axis and the absence of similar transformations when the magnetic field vector is perpendicular to the c axis. This anisotropic behavior is due to variation of magnetization energy between the c axis and the a or b axes of the orthorhombic crystal caused by a different alignment of the Gd moments with respect to the magnetic field vector. The anisotropy of the antiferromagnetic state diminishes with the increasing magnetic field and temperature. The critical magnetic field for the antiferromagnetic-ferromagnetic transition is the smallest and the ferromagnetic state is most stable when the magnetic field vector is parallel to the b axis, indicating an easy magnetization direction along this axis. The anisotropy of the magnetic field-induced transformation in Gd5Ge4 is discussed in connection with the coupled magnetic and structural transitions. Anisotropic magnetic phase diagrams along the three major crystallographic axes are constructed.