Revelation 7:1

The phrase "after this" (Gr. meta touto) indicates that what follows is a new vision (cf. Revelation 4:1). The general chronological progression of the visions suggests that the events John saw now will happen at the end of the first half of the Tribulation. What John saw in this vision supports that conclusion. [Note: Moffatt, 5:394.]

The angels described here were God’s agents (cf. Hebrews 1:14). They appear to be different from the four living creatures and the 24 elder-angels (if they are angels). Four of them received the task of keeping the wind from blowing. God stationed them at the four "corners" (i.e., compass points) of the earth for this purpose (cf. Revelation 20:8; Isaiah 11:12; Jeremiah 49:36; Matthew 24:31). The winds represent God’s judgments coming on the world (Revelation 7:3; cf. Jeremiah 49:36-38; Daniel 7:2; Hosea 13:15), specifically those about to follow during the remainder of the Tribulation (cf. Ezekiel 9:4-8). The threefold repetition of "four" probably stresses the universal control of these angels. [Note: Scott, p. 163; Smith, A Revelation . . ., p. 128.] We should probably understand the "sea" and any "tree" literally in view of what follows (cf. Revelation 7:3).