Last year I was studying supersymmetry and since them I'm regularly thinking to one question for which I don't have answer: when one looks at the susy algebra with N generators, one sees that there is a U(N) R-symmetry. But for N=4, the group is in fact SU(4). To explain this, one generally argues (Terning's book, McGreevy's lectures, problem set 2) using the form of the lagrangian, e.g. that the scalars are in the real antisymmetric 6 representation, and so a phase transformation is clearly not a symmetry of the lagrangian. But I'm remember to have read somewhere a computation at the level of the algebra (with N generators) which leads to a term with (N-4) and this explains why the case N=4 is special. Unfortunately I'm not able to remember the steps of the computation or to find again the place where I have read this; it's not in usual books (Terning, Weinberg, Wess/Bagger, West, Binétruy, Freund, Müller-Kirsten/Wiedemann) nor reviews (Sohnius, Bilal, etc.). Does someone have an idea or a hint about this?