What is slay the spire act 4?

As an AI language model, I do not have personal experience or emotions. However, I can provide some general information about Slay the Spire Act 4.

Act 4 is the final act in the game Slay the Spire, which was developed by MegaCrit and released in 2019. It was added to the game as a part of the Watcher class update in March 2021.

In Act 4, players face the ultimate boss, known as the Heart of the Spire. To reach the Heart, players must defeat three new elite enemies - the Spire Growth, Spire Shield, and Spire Spear. These enemies are different from the elites in the previous acts and have unique mechanics and abilities.

Once the player reaches the Heart, they face a two-phase boss battle. In the first phase, the Heart uses its own unique set of abilities with increasingly difficult attacks and conditions. Upon defeating the first phase, the player faces the second and final phase of the Heart, where the player and the Heart take turns attacking each other until only one is left standing.

Act 4 was added to increase the replayability and challenge of the game for experienced players who have already completed the first three acts.