  We calculate the radiative corrections to the Dalitz plot of K_{l3}^\pm
decays to order (alpha/pi)(q/M_1), where q is the momentum transfer and M_1 is
the mass of the kaon. We restrict the analysis to the so-called four-body
region, which arises when no discrimination of real photons is made either
kinematically or experimentally. We present our results in two ways: the first
one with the triple integration over the photon kinematical variables ready to
be performed numerically and the second one in a fully analytical form. Our
results can be useful in experimental analyses of the Dalitz plot, by
evaluating the model-independent coefficients of the quadratic products of the
form factors; we provide some numbers as a case example. We find a small,
albeit non-negligible, contribution from the four-body region to the radiative
correction to the total decay rate of K_{l3}^\pm decays.
