Can’t get from completed level three to level four. It demands I decorate, but that doesn’t unlock..

To advance, you have to add more Flowers to your island. The number of flowers you need is shown in red, next to the flower symbol in the top right corner. One of the ways to add more Flowers is to buy decorations, but be aware that you have to wait for the construction to be complete and remove the red bow, before the Flower value counts. Other ways to add Flowers will become available later in the game and include moving decoration from storage to island, upgrading buildings, renovating the landmark, and sending Jack on a delivery with flower rewards.