A 4-D triangle is called a simplex. It is made of 5 tetrahedra (five 3D triangles, count 'em!) Just like a triangle is made of 3 sides, tetrahedron (3D triangle) is made of 4 triangles joined edge-to-edge, and a simplex is made of 5 tetrahedra joined face to face. See the pattern? This is a "shadow" of a simplex, projected from 4 dimensions into our 3D "hyper-plane." You can also make a bubble model of this!