Gameplay Changes
This installment of the series introduces two new features: the pulley system and chain reactions. The pulley system consists of two groups of bubbles attached to either side of a pulley. Popping some on one side will cause that side to be "lighter" and therefore rise. The other side lowers in response. This requires added strategy to prevent one side moving too far and therefore losing the game.
Chain reactions occur only on the two player (or player and CPU) modes. When a bubble is dropped, it can move to another place on the board if this causes more bubbles to pop. If this in turn causes more bubbles to drop, then the chain reaction can continue.
Read more about this topic: Puzzle Bobble 4