The Four Horses are Four Judgements, Winds and Angels

In the first 4 seals, 4 horses are described.

Revelation 6:1 Then I looked on as the Lamb opened one of the seven seals. I heard one of the four living creatures say in a voice like thunder, "Come!" 2 So I looked, and there was a white horse. Its rider held a bow and was given a crown. And he went forth from victory to victory.
3 When the Lamb opened the second seal, I heard the second living creature say, "Come!" 4 Out came another horse, fiery red. Its rider was allowed to take peace from the earth so that people would kill each other. He was given a large sword.
5 When he opened the third seal, I heard the third living creature say, "Come!" So I looked, and there was a black horse. Its rider held a balance for weighing in his hand. 6 I heard what sounded like a voice from among the four living creatures. It said, "A quart of wheat for a denarion and three quarts of barley for a denarion, but don’t damage the olive oil and the wine."
7 When he opened the fourth seal, I heard the voice of the fourth living creature say, "Come!" 8 So I looked, and there was a pale green horse. Its rider’s name was Death, and the Grave was following right behind. They were given authority over a fourth of the earth, to kill by sword, famine, disease, and the wild animals of the earth.

In verse 8 it is explained what these horses represent: the sword (red horse), famine (black horse), disease (pale horse) and wild animals (white horse). In Deuteronomy 32:23-24 these four are mentioned as well as judgments that will come over Israel if they stop following the Lord.

In Zechariah 6 we find four similar colored horses like that. There it says:

Zechariah 6:5 "These are the four winds of heaven that are going out after presenting themselves to the LORD of all the earth.

In chapter 6 these four horses are announced, but in 7:1 they are hold back by four angels.

Revelation 7:1 After this I saw four angels standing at the four corners of the earth. They held back the earth’s four winds so that no wind would blow against the earth, the sea, or any tree .2 I saw another angel coming up from the east, holding the seal of the living God. He cried out with a loud voice to the four angels who had been given the power to damage the earth and sea. 3 He said, "Don’t damage the earth, the sea, or the trees until we have put a seal on the foreheads of those who serve our God."

Note that you can only truly understand this after you read in Zechariah that these four horses are four winds.
Also note that there are eight angels here. Four that are holding back the four winds. And four that are the winds in verse one, but are called angels in verse two. So the four winds are four angels as well.

Now the question is: when are they going to be released? You can read about that in 9:13

Revelation 9:13 Then the sixth angel blew his trumpet, and I heard a voice from the four horns of the gold altar that is before God. 14 It said to the sixth angel, who had the trumpet, "Release the four angels who are bound at the great river Euphrates." 15 Then the four angels who had been made ready for that hour, day, month, and year were released to kill a third of men. 

Here the four horses, judgements or winds are also called angels, as in 7:2. The four horses are released at the sixth seal. The fifth seal is about the siege of Jerusalem. The sixth seal is about the destruction of Jerusalem.