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.
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. Whe the Death Star arrived in the system, The Alliance sent X-Wings and Y-Wings to destroy the station. Grand Moff Tarkin was convinced off the station's invincibility and did nothing. Darth Vader however realized the danger and went out to stop the Rebels. Vader was about to blast Luke out of the air when Han Solo blasted him away. Luke fired into the port and destroyed the Death star winning the Battle of Yavin for the Rebel Alliance.