Classifying spaces of degenerating mixed Hodge structures, IV: The fundamental diagram

We complete the construction of the fundamental diagram of various partial compactifications of the moduli spaces of mixed Hodge structures with polarized graded quotients. The diagram includes the space of nilpotent orbits, the space of SL(2)-orbits, and the space of Borel--Serre orbits. We give amplifications of this fundamental diagram, and amplify the relations of these spaces. We describe how this work is useful to understand asymptotic behaviors of Beilinson regulators and of local height parings in degeneration. We discuss "mild degenerations" in which regulators converge.