SPAG4D is a production-grade tool that converts 360° equirectangular panoramic photos into explorable 3D Gaussian Splat (3DGS) scenes. It integrates multiple depth estimation backends (DA360, DAP, PaGeR, SHARP, UniSHARP) to generate depth maps from a single panorama, then projects the points into standard 3DGS PLY format. The system provides a CLI, a FastAPI web server with a JavaScript frontend, and a Python API for programmatic use. Beyond the core conversion, it includes optional refinement pipelines to fill holes and improve scene quality: OmniRoam (WSL2-based video generation and distillation), ArtiFixer3D (Docker-based diffusion refinement), and a lighter geometric refinement pipeline using depth alignment and voxel aggregation. The architecture is modular with pluggable generators, comprehensive configuration dataclasses, and provenance tracking. While the tool is primarily an application, it leans heavily on research-grade depth models and refinement techniques, giving it a hybrid character. The codebase is well-structured with clear separation of concerns, reusable utilities (spherical projection, depth blending, scene filtering, geometric alignment), and extensive tests. It is actively maintained with recent commits, though it carries complexity from multiple backends and platform-specific dependencies (WSL2, Docker, CUDA).