We characterize (both from a syntactic and an algebraic point of view) the normal K4-logics for which unification is filtering. We also give a sufficient semantic criterion for existence of most general unifiers, covering natural extensions of K4.2+ (i.e., of the modal system obtained from K4 by adding to it, as a further axiom schemata, the modal translation of the weak excluded middle principle).
"Filtering unification and most general unifiers in modal logic." J. Symbolic Logic 69 (3) 879 - 906, September 2004. https://doi.org/10.2178/jsl/1096901773