One popular slogan Armstrong referred to was often quoted by the crowd is you have to let the Bible interpret the Bible. If a person assumes that is the case 100% of the time, then by the reasoning you've presented, you are correct and what was taught about the four horsemen in that church is in error. Someone else also mentioned Revelation 19:11, a verse that undeniably refers to Jesus as sitting on a white horse in the same book. I'm not sure why people thought that letting the Bible interpret the Bible was a fool proof method of coming up with interpretations.
However I believe what ViaCrusis concludes is closer to what can be implied about what the horses represent. They are symbols used to describe the condition of civilization on the earth.
Since the beginnings of the NT era Christianity as a whole has had its' controversies. Jude 1:12 Acts of the Apostles 20:30 2 Corinthians 11:4 So just like we see what is mostly a common state of war, famine and death, Christianity has also had an influence on this world. I think that fourth horse is a symbol of what is going on within churches across the globe, the good and the bad. The reason why in Rev 19:11 is clear about who is riding a white horse is to place contrast between the two white horses and not to show that they are the same.
One shouldn't discard stuff just because a false prophet uses it; false prophets use every verse in the Bible; would you discard the Bible? The reason I wouldn't call Armstrong a false prophet is I wouldn't know where to stop; I'd end up calling all religions false.
Armstrong is not the only one to say the Bible interprets itself; the Bible does interpret itself in many ways. In the case of the four horses Armstrong does not use the Bible to interpret them; if he did he would have used Zechariah 1:8-11.
Zechariah 1:8-11 (NKJV)
8 I saw by night, and behold, a man riding on a red horse, and it stood among the myrtle trees in the hollow; and behind him
were horses: red, sorrel, and white.
9 Then I said, "My lord, what
are these?" So the angel who talked with me said to me, "I will show you what they
are."
10 And the man who stood among the myrtle trees answered and said, "These
are the ones whom the LORD has sent to walk to and fro throughout the earth."
11 So they answered the Angel of the LORD, who stood among the myrtle trees, and said, "We have walked to and fro throughout the earth, and behold, all the earth is resting quietly."
While Rev. mentions Satan it is not about Satan directly; Rev is a transcript of that great and terrible day of the Lord also call the Lord's day, not that great and terrible day of Satan.