Yavin 4 is a rainforest moon orbiting its gas giant, Yavin 4 also has a secret Rebel base. It's also where the Battle of Yavin took place.
History
Anakin Skywalker met and fought Asajj Ventress for the first time on Yavin 4. Years later, the Rebel Alliance established a base here. When Leia Organa returned with the plans to the Death Star, they found that they could destroy it by firing into an unsheilded exhust port.