The first three horses that were continuously running came in between his line of sight with the fourth horse many times.
Just because he had already shot those three horses, doesn't mean that they would leave the ground or would be solved in any other way.
On the contrary they would keep on running making it even more difficult to aim the target board on the horse that was running in the outer ring.
The more outer rings one approached, the more difficult the level would be.
This difficulty was not constrained with just the increased speed compared to the ones in the inner rings but also the fact that there are many 'obstacles' that would run around in between the archer and his target.
Now not only should the Seventh Prince aim his arrow towards the target on the fourth horse, but he should also avoid accidentally hitting any of the three horses that would run in front of the fourth horse.
