A note on the geometry of linear Hamiltonian systems of signature 0 in   R^4
  It is shown that a linear Hamiltonian system of signature zero in 4 dimensions is elliptic or hyperbolic according to the number of Lagrangian planes in the null-cone $H^{-1}(0)$, or equivalently the number of invariant Lagrangian planes. An extension to higher dimensions is described. 
James Montaldi