Finite basis problem for involution semigroups of order four

Recently, we have found a non-finitely based involution semigroup of order five. It is natural to question what is the smallest order of non-finitely based involution semigroups. It is known that every involution semigroup of order up to three is finitely based. In this paper, it is shown that every involution semigroup of order four is finitely based. Therefore, the minimum order of non-finitely based involution semigroups is five.