We report the results of measurements of the electrical resistivity, the magnetization, and the specific heat of PrB4 below room temperature. The crystal electric field (CEF) splits the 4f2 multiplet of the Pr ions into nine singlets. In its ground state, PrB4 is an induced-moment ferromagnet with a saturation magnetization as high as 2.1μB (Prion). Contrary to previous experiments, we observe a first-order ferromagnetic to antiferromagnetic transition at TC≈15.9±0.1K with a latent heat ΔQ=8.6J mol. The antiferromagnetic long-range order is quenched above TN=19.5K. Susceptibility and specific heat data, as well as the enormous magnetic anisotropy, can quantitatively be reproduced by a mean-field calculation which takes into account the CEF, obtained from a point-charge model using effective charges obtained from a self-consistent electronic band-structure calculation and a Zeeman term, as well as an anisotropic dipolar magnetic exchange and an effective quadrupole-quadrupole interaction term. © 2005 The American Physical Society.