Revelation chapter 6 presents what are commonly understood to be the four horsemen of the apocalypse. We are told in the text that there are four horses and their respective colors are given but we are never actually told that there are four riders.
Recognizing that the horses are symbolic and that riders control or direct the horses, could it be that the riders are controlling/directing whatever the horses represent? Also, there is no need to be concerned over the question of whether a rider can literally ride more than one horse at a time because they are not literal horses. There is considerable evidence that the horses represent different groups of people. Here are a couple of many examples:
"... the LORD of hosts hath visited his flock the house of Judah, and hath made them as his goodly horse in the battle." (Zech 10:3)
"I have compared thee, O my love, to a company of horses in Pharaoh’s chariots." (S.Sol. 1:9)
We looked into this in our Sabbath services as we continue to work through the book of Revelation. We saw evidence that these four groups corresponded to the four horses of Zechariah 6 and the four categories mentioned in Revelation 22:11. Our conclusion was that Revelation 6 is depicting four groups of people each under the direction of either Christ or Satan, the principle protagonists in the great controversy. So really, there are only two riders, two horsemen of the apocalypse. All the details can be read at www.jesus-resurrection.info/revelation-chapter-6.html.