Yep, flag 4 is a bodge. To do flat shading properly, you need surface normals to light it, which blitz meshes (or is it DX?) doesn't support. Flag 4 simply causes each triangle to be lit using one of it's vertex normals, as if it were a surface normal. Unfortunately, vertex normals are usually smoothed and so will not be perpendicular to the triangle.
John, if you simply want to load a mesh in your app and have it display using flat shading, then you're out of luck, I'm afraid. Your only real option is to do what Peter says and rebuild the mesh.
John, if you simply want to load a mesh in your app and have it display using flat shading, then you're out of luck, I'm afraid. Your only real option is to do what Peter says and rebuild the mesh.