The fourth round was extremely hard to cross for anyone who was not extremely proficient with magic and also had decent power behind it. Only having one of those two qualities would not be enough, a mage needed to have both to cross this round.
It was a fight against a special kind of mechanical puppet. This puppet was a S rank puppet so none of the participants needed to defeat as it would be almost impossible. What they needed to do was to fulfill a few conditions against this boss to go to the next round.
This puppet was designed in such a way that it fought using a few patterns. Firstly the mage who was fighting it needed to figure out the magic pattern which this puppet was using at that time. With that the mage would be able to guess how the puppet was going to attack next and would be able to counter that accordingly.