Pseudo-Riemannian almost hypercomplex homogeneous spaces with   irreducible isotropy
  We classify homogeneous pseudo-Riemannian manifolds of index 4 which admit an invariant almost hyper-Hermitian structure and an H-irreducible isotropy group. The main result is that all these spaces are flat except in dimension 12. 
Vicente Cort\'es, Benedict Meinke
math.DG