Simultaneous binary collisions in the planar four-body problem

We consider simultaneous binary collisions in the general planar four-body problem. We prove they are regularizable in the sense of continuity with re spect to initial conditions using a blow-up of the singularity. Furthermore , numerical evidence is given that the differentiability of the regularizar ion should be, in general, less than C-8/3. As a Simple example, the double isosceles four-body problem, displays that kind of behaviour.