The term translated here as “holy” is not the standard word for holy (one that denotes the notion of separateness and hence of purity), but one that is rarely used (in Revelation only in Revelation 16:5). It seems to catch the notion of keeping commitments. In this instance, then, the term describes God as acting according to his promises. His plagues (= deeds, ways) do not come out of nowhere but are in step with what he had said he would do.
4 Who will not fear, O Lord, and glorify your name? For you alone are holy. All nations will come and worship you, for your righteous acts have been revealed.”