We prove that the 824 link
complement is the minimal volume orientable hyperbolic manifold with 4 cusps. Its
volume is twice the volume V8 of the ideal regular octahedron; that is, 7.32…= 2V8.
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 nonseparating boundary.