We analysed some qualitative properties of the limit configuration of the
solutions of a reaction-diffusion system of four competing species as the
competition rate tends to infinity. Large interaction induces the spatial
segregation of the species and only two limit configurations are possible:
either there is a point where four species concur, a 4-point, or there are two
points where only three species concur. We characterized, for a given datum,
the possible 4-point configuration by means of the solution of a Dirichlet
problem for the Laplace equation
