Full pion four-point functions at zero momentum are calculated with Kogut-Susskind quarks on a 123×20 lattice at β=5.7 and mq=0.01 in quenched lattice QCD, employing the wall source method without gauge fixing. For both I=0 and 2 channels, results for the s-wave scattering lengths are in agreement with the prediction of current algebra and PCAC (partial conservation of atral-vector current).