Clearly, this is God himself (see also Revelation 4:1–2; Revelation 4:10; Revelation 5:1). This would suggest that the words spoken in Revelation 21:5 come from a different speaker than the words of Revelation 21:3–4. As the present words come directly from God (as opposed to indirectly, i.e., via an angel or a living creature) they come with increased weight (though that is not at all to say that the previous words may be ignored!) and demand focused attention.
5 And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.”