Taken from the official announcement:

Executive Producer George Lucas and Lucasfilm Animation present Star Wars: The Clone Wars The Complete Season Four. The battle to save the galaxy reaches new heights of peril and new depths of darkness. The valiant Jedi Knights and clone troopers defend exotic planets from enemy forces. They wage war underwater, battle across a night-shrouded world, and take on the merciless Death Watch. Our heroes are tested like never before as Anakin is haunted by the realities of his past, Ahsoka fights to free her people and an undercover Obi-Wan braves a deadly bounty hunter tournament. Season Four culminates with the shocking return of a villain from the past: the ruthless Darth Maul, who will stop at nothing to achieve vengeance. The epic action, timeless characters, and sense-shattering sights and sounds of Star Wars surge on in Star Wars: The Clone Wars The Complete Season Four.

Special Features:
22 Adventure-Packed Episodes

5 Video Commentaries

Exclusive to Blu-ray for a Limited Time: Jedi Temple Archives: Extensive Database of Early Test Animations, Concept Art & 3-D Turnarounds

UPDATE:
IGN have posted the Star Wars: The Clone Wars The Complete Season Four trailer and have also provided the following information on five video commentaries included on the set that feature supervising director Dave Filoni and other cast and crew.

The Battle of Mon Cala: Dive into the depths of animation production as Supervising Director Dave Filoni and key members of his crew discuss the challenges of creating this complex series of episodes, full of underwater warriors and deep sea action.

Darkened World of Umbara: Supervising Director Dave Filoni and key members of his crew shine a light on the shadowy battlefields of Umbara, discussing the technical hurdles in creating this epic arc, as well as the storytelling challenges of placing Captain Rex and his loyal soldiers in this most difficult of situations.

Obi-Wan Undercover: The truths behind the deception are revealed as Supervising Director Dave Filoni and key members of his crew chat about the creation of this storyline full of betrayal, reversals, and shady underworld figures.

Slaves of Zygerria: Supervising Director Dave Filoni and key members of his crew discuss the development of a story that began life as a comic book before being transformed into an action-packed animated drama.

Darth Maul Returns: Darth Maul is resurrected to become a new menace in The Clone Wars, and Supervising Director Dave Filoni and key members of his cast and crew discuss how this shocking turn of events came to be, and what it took to bring the Sith warrior back to the screen.

The following extra feature is exclusive to the Blu-ray version:

The Jedi Temple Archives: An extensive database exploring special effects footage, early concept art, 3D character and object turnarounds, early animation and more than 20 deleted/extended scenes. The Archives is a rare look at the assets in various stages of creation for The Clone Wars, with a wealth of surprises and never-before seen moments sprinkled throughout

Star Wars: The Clone Wars The Complete Season Four is presented in widescreen with English, Spanish and French 5.1 DTS audio and English, French and Spanish subtitles.