High-precision estimate of g4 in the 2D Ising model
  We compute the renormalized four-point coupling in the 2d Ising model using transfer-matrix techniques. We greatly reduce the systematic uncertainties which usually affect this type of calculations by using the exact knowledge of several terms in the scaling function of the free energy. Our final result is g4=14.69735(3). 
Michele Caselle, Martin Hasenbusch, Andrea Pelissetto and Ettore   Vicari