See {@link com.carrotsearch.ant.tasks.junit4.JUnit4} for the documentation of available task options.
Listeners and reporters API is covered in {@link com.carrotsearch.ant.tasks.junit4.listeners.AggregatedEventListener} class. Note that the API is event-based and uses Google Guava's {@link com.google.common.eventbus.EventBus} to distribute events. Check out existing reporters for implementation details:
Examples of use and more resources can be found at: Carrot Search labs site.