The fourth dungeon is the Guild LLC, the Central Bank of the island.

The Guild

You can talk to the skeletons here to hear some bad puns. If you go up the ladder on the left, you can play the trading minigame. If you go down the ladder on the right, then go to the right, and the path will lead you to the chairman. After some dialog, you will enter the fourth dungeon.

Find the Treasure

As usual, find the treasure of the dungeon, then optionally beat the boss. If you leave the dungeon after getting the treasure, you get another call from Prince Daniel. You will receive the Power of Compassion.

You will also encounter the Chairman as you leave the Guild building. You can go into his office to take the money that he left behind.

After this, your map will be marked with your next destination, the bridge. The chairman will attack you by throwing coins at you, which you can deflect with your shield. Get close to him and there will be some dialogue. Then go east and you will meet Delvin. The next dungeons will be marked on your map.