I dreamt there were four knights on horseback riding ahead of me. We didn't seem to be anyplace in particular, just open ground. I guess I was riding, too, but I couldn't really tell. I didn't look down or try to see myself. I just wondered what these knights could mean.
Okay, well this is pretty open, so it could mean a lot of things. Most of it will depend on how you felt in the dream? If you were anxious, if you felt like you were being held prisoner or going against your will, that's one thing. If these knights were protecting you, if you felt safe, that's something else. Or maybe they were heralding you, which is something else yet again.
So let's go over the various possibilities. Knights typically symbolize protection of some kind, as in the classic "knight in shining armor." There may be a romantic aspect to the dream in that your subconscious is yearning for that kind of romance or protection, even a rescue of some kind. If you were chasing the knights, rather than just riding behind them, this aspect is emphasized.
You were riding behind them; they weren't surrounding you. This could mean they are going ahead to lead the way to something. They could be guides. Or maybe they are heralds, going ahead to announce your arrival. In this aspect, the dream might be about coming success, or a wish to succeed. Even a wish to be noticed.
Colors and symbols are important in dreams, so if the horses were a particular color that might have a bearing on the dream's meaning. White horses would again skew toward that romantic idea, black horses might mean something darker, brown horses are more earthy and grounded and would have more to do with the physical/material world.
If the knights were carrying anything—lances, swords—these, too, would need to be taken into account. In Tarot, there are knights and there are four suits: Wands, Pentacles (or Coins), Swords, and Cups. Four wands would be a good omen; four swords would mean a need to retreat or withdraw, to rest and regroup.
That the land was, as I understand it, empty is interesting because it suggests an open playing field. No obstacles. If you had a sense of where you were headed in the dream, that might be helpful in determining the overall meaning.