  We explain a direct topological proof for the multiplicativity of Duflo
isomorphism for arbitrary finite dimensional Lie algebras, and derive the
explicit formula for the Duflo map. The proof follows a series of implications,
starting with "the calculation 1+1=2 on a 4D abacus", using the study of
homomorphic expansions (aka universal finite type invariants) for ribbon
2-knots, and the relationship between the corresponding associated graded space
of arrow diagrams and universal enveloping algebras. This complements the
results of the first author, Le and Thurston, where similar arguments using a
"3D abacus" and the Kontsevich Integral were used to derive Duflo's theorem for
metrized Lie algebras; and results of the first two authors on finite type
invariants of w-knotted objects, which also imply a relation of 2-knots with
Duflo's theorem in full generality, though via a lengthier path.
