Simultaneous binary collisions in the planar four body problem Regina Mart\'{\i}nez, Departament de Matem\`atiques Universitat Aut\`onoma de Barcelona, Bellaterra, Barcelona, Spain Carles Sim\'o, Dept. de Matem\`atica Aplicada i An\`alisi, Univ. de Barcelona, Gran Via 585, 08007 Barcelona,Spain Abstract We consider simultaneous binary collisions in the general planar four body problem. We prove they are regularizable in the sense of continuity with respect initial conditions using a blow up of the singularity. Furthermore, numerical evidence is given that the differentiability of the regularization should be, in general, less than $ C^{8/3} $. A simple example, the double isosceles four body problem, displays that kind of behaviour.