After this (the 6 seals opened) I saw four angels standing at the four corners of the earth, holding back the four winds of the earth,

However, while this is a physical event, this verse may also be symbolic of withholding judgment. In Isaiah 27:7-11, we find that "the east wind" means exile for Israel. This is a softening process on the hearts of the people. In Matthew 7:24-27, a wind blew (or trouble came) against the house. The scorching wind in Jeremiah 4:10-13 implies an invading army or marauding bands of thugs would torment Israel! Daniel in Daniel 7:1-8 uses the phrase of "the four winds of heaven" stirring up four world empires (beasts). This cannot be good.

Also consider that the sixth seal being broken symbolicly shows a time of GREAT persecution of believers during the tribulation. False doctrines can blow around men who are not grounded in the Word of God. Now a calm period permits the earth inhabitants to ground themselves deeper for what lies ahead.

so that no wind should blow on the earth or on the sea or on any tree.

If this is literally true, a different mechanism to cause it to rain on the earth must be instituted. That is, as we know it, water evaporates from the sea, the wind moves it over land, and it rains. Since the wind is stopped, this cycle is stopped. To not harm the trees (verse 3), this period must be short. Otherwise the trees would die.