On Geometry and Topology of 4-Orbifolds
  We prove an analogue of the result of Hsiang and Kleiner for 4-dimensional compact orbifolds with positive curvature and an isometric circle action. Additionally, we prove that when the underlying space is simply connected, then the orbifold fundamental group provides a bound on the failure of integer-valued Poincare Duality of the underlying space, and if the orbifold is simply connected, then ineteger-valued Poincre Duality holds for the underlying space. 
Dmytro Yeroshkin
math.DG math.AT math.GT