A two-sublattice spin model is proposed with special four-spin interactions besides the usual exchange terms. The four-spin interaction terms are assumed not to be the even functions of the spin sublattice operators. Thermodynamical properties of this model are investigated in the molecular field approximation. The complete phase diagrams for all experimentally discovered collinear two-sublattice metamagnets are obtained numerically. Moreover some unknown kinds of phase diagrams are found, determining the new types of metamagnets which have not been experimentally observed so far. Finally, the theoretical results fitted for the FeCl2 and Mn3GaC, are compared with experiment.
