  We propose a general method (based on the Wang-Landau algorithm) to compute
numerically free energies that are obtained from the logarithm of the ratio of
suitable partition functions. As an application, we determine with high
accuracy the order-order interface tension of the four-state Potts model in
three dimensions on cubic lattices of linear extension up to L=56. The infinite
volume interface tension is then extracted at each beta from a fit of the
finite volume interface tension to a known universal behavior. A comparison of
the order-order and order-disorder interface tension at the critical value of
beta provides a clear numerical evidence of perfect wetting.
