  We prove that the 8^4_2 link complement is the minimal volume orientable
hyperbolic manifold with 4 cusps. Its volume is twice of the volume V_8 of the
ideal regular octahedron, i.e. 7.32... = 2V_8. The proof relies on Agol's
argument used to determine the minimal volume hyperbolic 3-manifolds with 2
cusps. We also need to estimate the volume of a hyperbolic 3-manifold with
totally geodesic boundary which contains an essential surface with
non-separating boundary.
