Albert Barnes on Revelation 6:7

And when he had opened the fourth seal. Rev 5:1.

I heard the voice of the fourth beast say. The flying eagle. Rev 4:7. As in the other cases, there does not appear to have been any particular reason why the fourth of the living creatures should have made this proclamation rather than either of the others. It was poetic and appropriate to represent each one in his turn as making proclamation.

Come and see. Rev 6:1.