Mn3O4 is one of the archetypal examples of tetragonally distorted spinels. In this paper we reinvestigate its complex magnetic ordering process by using high-resolution magnetic and calorimetric measurements. The first-order transition to the ground state is found to exhibit a latent heat of 4 J mol-1 and a thermal hysteresis of 0.6 K. Moreover, the peculiar field-induced broadening observed at this transition is shown to be consistent with general predictions about incommensurate-to-commensurate transition. In addition to the known sequence of transitions at TN43 K, T140 K, and T234 K, a new anomaly close to T38 K is observed in both magnetization and heat capacity data. A possible origin of this feature is discussed in terms of magnetic instability within the incommensurate spiral spin configuration.