Albert Barnes on Revelation 6:3

And when he had opened the second seal. So as to disclose another portion of the volume. Rev 5:1.

I heard the second beast say. The second beast was like a calf or an ox. Rev 4:7. It cannot be supposed that there is any special significancy in the fact that the second beast addressed the seer on the opening of the second seal, or that, so far as the symbol was concerned, there was any reason why this living creature should approach on the opening of this seal rather than on either of the others. All that seems to be designed is, that as the living creatures are intended to be emblems of the providential government of God, it was proper to represent that government as concerned in the opening of each of these four seals indicating important events among the nations.

Come and see. Rev 6:1.