By performing the calculation ``$1+1=2$'' on a 4D abacus, we explain in the most direct way we know how the study of "expansions", or "universal finite type invariants", for ribbon 2-knots leads to a proof of Duflo's theorem for arbitrary finite-dimensional Lie algebras. This complements the results of B-N, Le, and Thurston~\cite{Bar-NatanLeThurston:TwoApplications} where a similar argument using a 3D abacus and the Kontsevich integral was used to deduce Duflo's theorem yet only for metrized Lie algebras, and our results from~\cite{WKO2} which also imply a relation of 2-knots with the full Duflo theorem, though via a lengthier path.