Several characterizations of the 4--valued modal algebras

A. Monteiro, in 1978, defined the algebras he named tetravalent modal algebras, will be called 4--valued modal algebras in this work. These algebras constitute a generalization of the 3--valued Lukasiewicz algebras defined by Moisil. The theory of the 4--valued modal algebras has been widely developed by I. Loureiro in [6,7,8,9,10,11,12] and by A. V. Figallo in [2,3,4,5]. J. Font and M. Rius indicated, in the introduction to the important work [1], a brief but detailed review on the $4-$valued modal algebras. In this work varied characterizations are presented that show the "closeness" this variety of algebras has with other well--known algebras related to the algebraic counterparts of certain logics.