What was the name of the rider of the pale horse at the fourth seal?

Answer: Death

Its rider's name was Death, and Hades followed close behind.

“Then I looked and saw a pale green horse. Its rider’s name was Death, and Hades followed close behind. And they were given authority over a fourth of the earth, to kill by sword, by famine, by plague, and by the beasts of the earth.”— Revelation 6:8 (BSB)