The 600-cell's 4D coordinates upload to the GPU once. Each frame, the vertex shader rotates them in 4D and performs the perspective divide — the CPU sends only a 4×4 matrix. This works because a 4D rotation is a linear map on exactly 4 numbers: it fits the GPU's native mat4 like a glove.
Toggle the CPU check overlay: the blue wireframe is the same object recomputed on the CPU in double precision. Solid blue means both paths agree to sub-pixel — the differential test that keeps the fast path honest. Faces (GPU) adds the 1200 triangles, also for the price of one matrix.