What is the fourth layer when dealing with continuous effects?

The fourth layer in the framework for applying continuous effects in Magic: The Gathering deals specifically with type-changing effects. This layer is essential when determining how a permanent's characteristics can change during the game based on the effects currently in play.

To understand this better, it's important to recognize the sequence of layers in the rules, particularly the order in which different modifications to a permanent are applied. The first layer deals with defining the characteristics of a card, such as its base power and toughness, while the subsequent layers address other effects that may modify those characteristics, including which creature types a permanent may belong to.

Type-changing effects apply after color-changing effects and before text-changing effects. For instance, when a creature becomes a different type (like from a Human to a Zombie), that change is registered in this fourth layer, influencing how that permanent interacts with other cards, abilities, and effects in the game context.

While text, color, and control-changing effects are also important and affect a permanent's characteristics, they specifically fall in different layers according to the rules structure. Understanding this layering is crucial for making accurate rulings and for players to grasp how their cards will function under various continuous effects during a game.