The four high-paying symbols are the so-called Agents. When you land a cluster with one of them, it fills the respective meter next to the Agent of Hearts grid. It takes 6 Agent clusters of the same type to launch the special bonus feature. Another way to launch the bonus features is by seeing the White Rabbit, who will activate a random Agent.
Here is a quick rundown of the features:
Cupcake Splat is the Dormouse bonus. The character will move from top to bottom, leaving a trail of cupcakes that remove or transform the symbols beneath them.
Smokin' Phat is the Caterpillar’s feature. The Agent will blow 1 to 3 trails, all diagonally. The affected icons will be transformed or removed.
Grinning Cat is a feature in which the Cheshire Cat is the main Agent. The Agent will grin, and all symbols beneath the affected positions will either be removed or transformed.
Mad Hat is, naturally, related to the Mad Hatter. It will unleash 6 to 9 Mad Hatters, who will remove all adjacent symbols and replace them with matching, randomly chosen symbols.