Many say that the rider of the first seal horse in rev6 is the antichrist, but the riders of the second third and fourth seal judgment horses are not often given.
The book with the seven seals is in Heaven, and Jesus is the one who opens these seals of God's judgments.
God's trumpet and vial judgments are carried out by God's Angels, Angels sound the trumpets of God's judgments and Angels pour out the vials of God's judgments. So it could be right to assume that Angels also carry out God's seal judgments, and are the riders of the seal judgment horses. Or the riders are perhaps just the judgments themselves.

 

Rev 6:7  And when he had opened the fourth seal, I heard the voice of the fourth beast say, Come and see.
Rev 6:8  And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth.

 

God Bless, John Too.