World 4, Limbo, is characterized by very few spots of safe ground, enemies that cannot be stomped on moving up-and-down or diagonally, and floating doodads that will allow you to recharge while keeping your combo intact, allowing for some very high combo values.
The background music in World 4 is Broken Ghost.
Note that cashing in combos at 25 is generally preferable sooner rather than later to allow you to begin a new combo. However, with so little safe ground this can be difficult on World 4, but be aware the very ledge of a safe zone bubble is considered outside the bubble, but still on the ground. So even safe zones without extended ledges (as shown) can be used to cash in a combo by walking off the ledge rather than jumping.