In this study, the porous substrate and stress control techniques were adopted to successfully break through the dislocation suppression and stress control problems in the growth of heterogeneous epitaxial GaN single crystals, and high-quality GaN single crystals with a diameter of 4-inch were prepared. After cutting, chamfering, grinding, and chemical-mechanical polishing, a damage-free, ultra-smooth 4-inch self-separating GaN single crystal substrate with a thickness of 500 μm was obtained. The substrate has both excellent crystalline quality and mechanical stability, with uniform surface color, no cracking phenomenon, and uniform stress distribution; cathodoluminescence spectroscopy (CL) measurements reveal a dislocation density of 9.6×10⁵ cm^-2, and the rocking curve of high-resolution X-ray diffraction (HRXRD) (002) is as low as 57.91″; the surface roughness Ra<0.2 nm measured by atomic force microscope (AFM), presenting atomic-level flat surface. The as-prepared substrate is ready-to-use, meeting the requirements for blue/green laser diodes and power electronic devices.