  We study the potential between static SU(4) sources using the Model of Thick
Center Vortices. Such vortices are characterized by the center elements
$z_1=\mathrm i$ and $z_2=z_1^2$. Fitting the ratios of string tensions to those
obtained in Monte-Carlo calculations of lattice QCD we get $f_2>f_1^2$, where
$f_n$ is the probability that a vortex of type $n$ is piercing a plaquette.
Because of $z_2=z_1^2$ vortices of type two are overlapping vortices of type
one. Therefore, $f_2>f_1^2$ corresponds to the existence of an attractive force
between vortices of type one.
