The fourth season of Code Lyoko originally aired from May 18, 2007 to November 17, 2007. It mainly dealt with the group traveling the web in search of X.A.N.A., in their virtual submarine: the Skidbladnir. Through their ship, they go searching for Replikas to destroy them to prevent X.A.N.A. from succeeding of world domination. They also try to rescue a X.A.N.A.-fied William and bring him back home.
Story
Recreating Lyoko and the Return of William
William returns as X.A.N.A.'s loyal minion
After the Core of Lyoko is destroyed, the group searches for a way to rebuild Lyoko once more so they can travel through the web. After a long summer, Jeremie and Aelita announce that they have successfully rebuilt Sector 5. The group also discusses William and how he was lost in the Digital Sea after Lyoko was destroyed; however, Jeremie has a trick up his sleeve and calls the principal to inform him that William would not be returning to Kadic Academy anytime soon. However, to add to the mystery, William mysteriously returns. This was simply a plot by X.A.N.A. to capture and kill Aelita. After a brief battle, the group decides that William is permanently X.A.N.A.-fied and they must rescue him. (William Returns)
The Skidbladnir
The Skidbladnir in the middle being fully constructed.
Since X.A.N.A. is now scattered around the internet, the team is now faced on how to counter X.A.N.A. As Jeremie works around the clock to find a solution, he finally comes up with the idea of a virtual submarine, named the "Skidbladnir". X.A.N.A. attempts to destroy the ship before it is fully completed. He fails in this attempt, however, he is able to steal some data which would be later used to program William's Rorkal. (Skidbladnir)
Battle of the Replikas
The Lyoko Warriors discover the Forest Replika.
With the Skidbladnir all programmed and ready to go, the Team decides to hunt down X.A.N.A. on the network as to bring William and Waldo Schaeffer home. In the maiden voyage, the team stumbles across an unknown sphere. However, the team later realizes that this was not in fact Lyoko. (Maiden Voyage)
They later return to this mysterious sphere and realize it is an exact copy of Lyoko, except that instead of housing four sectors, it houses just one. (Replika) These "Replikas" are housed the same way as Lyoko is: inside a supercomputer. During this time, Jeremie programs a way to bring them to the real world as spectres to destroy supercomputers that house the Replikas. (Lab Rat, Bragging Rights, A Lack of Goodwill,Hard Luck,A Space Oddity, Cousins Once Removed, Cold Sweat, Down to Earth)
Return of Franz Hopper
Franz Hopper.
Waldo Schaeffer (also known as Franz Hopper) makes his first re-appearance after Aelita is thrown into the Digital Sea. He saves Aelita, and also provides Jeremie with some data before disappearing once again. (Distant Memory)
Schaeffer appears for the last time in the penultimate-episode. He provides Jeremie with data that could potentially destroy X.A.N.A. (Fight to the Finish)
Demise of Franz Hopper and the Annihilation of X.A.N.A.
The Multi-Agent program in a working state.
With the data finally accessible, Jeremie programs the "Multi-Agent Program" which can destroy all of the Replikas and X.A.N.A. However there was not enough power available. As Mantas were attacking Franz Hopper, he offered the power to utilize the program.
Just as the program began to run, he was permanently devirtualized. X.A.N.A. was wiped out along with the Replikas, with the cost of Waldo's own life as well. (Fight to the Finish)
Episodes
| 1 (66) | 401 | William Returns |  |
Renaissance | US: May 18th, 2007 FR: August 13th, 2007 |
| 2 (67) | 402 | Double Take |  |
Mauvaise réplique | US: June 12th, 2007 FR: August 14th, 2007 |
| 3 (68) | 403 | Opening Act |  |
Première partie | US: June 19th, 2007 FR: October 29th, 2007 |
| 4 (69) | 404 | Wreck Room |  |
Double foyer | US: June 26th, 2007 FR: August 15th, 2007 |
| 5 (70) | 405 | Skidbladnir |  |
Skidbladnir | US: July 3rd, 2007 FR: August 16th, 2007 |
| 6 (71) | 406 | Maiden Voyage |  |
Premier voyage | US: July 10th, 2007 FR: August 17th, 2007 |
| 7 (72) | 407 | Crash Course |  |
Leçon de choses | US: July 17th, 2007 FR: August 20th, 2007 |
| 8 (73) | 408 | Replika |  |
Réplika | US: July 24th, 2007 FR: August 21st, 2007 |
| 9 (74) | 409 | I'd Rather Not Talk About It |  |
Je préfère ne pas en parler | US: July 31st, 2007 FR: August 22nd, 2007 |
| 10 (75) | 410 | Hot Shower |  |
Corps céleste | US: August 23rd, 2007 FR: August 14th, 2007 |
| 11 (76) | 411 | The Lake |  |
Le lac | US: August 21st, 2007 FR: August 24th, 2007 |
| 12 (77) | 412 | Lost At Sea |  |
Torpilles virtuelles | US: August 28th, 2007 FR: August 27th, 2007 |
| 13 (78) | 413 | Lab Rat |  |
Expérience | US: Unaired (Aired Online) FR: August 28th, 2007 |
| 14 (79) | 414 | Bragging Rights |  |
Arachnophobie | US: September 8th, 2007 FR: August 28th, 2007 |
| 15 (80) | 415 | Dog Day Afternoon |  |
Kiwodd | US: September 15th, 2007 FR: August 29th, 2007 |
| 16 (81) | 416 | A Lack of Goodwill |  |
Œil pour œil | US: September 22nd, 2007 FR: August 30th, 2007 |
| 17 (82) | 417 | Distant Memory |  |
Mémoire blanche | US: September 29th, 2007 FR: August 31st, 2007 |
| 18 (83) | 418 | Hard Luck |  |
Superstition | US: October 6th, 2007 FR: September 8th, 2007 |
| 19 (84) | 419 | Guided Missile |  |
Missile guidé | US: October 13th, 2007 FR: September 15th, 2007 |
| 20 (85) | 420 | Kadic Bombshell |  |
La belle de Kadic | US: November 3rd, 2007 FR: September 22nd, 2007 |
| 21 (86) | 421 | Canine Conundrum |  |
Kiwi superstar | US: November 10th, 2007 FR: September 29th, 2007 |
| 22 (87) | 422 | A Space Oddity |  |
Planète bleue | US: November 17th, 2007 FR: October 6th, 2007 |
| 23 (88) | 423 | Cousins Once Removed |  |
Cousins ennemis | US: November 17th, 2007 FR: October 13th, 2007 |
| 24 (89) | 424 | Music To Soothe the Savage Beast |  |
Il est sensé d'être insensé | US: Aired Online FR: October 20th, 2007 |
| 25 (90) | 425 | Wrong Exposure |  |
Médusée | US: Aired Online FR: October 27th, 2007 |
| 26 (91) | 426 | Bad Connection |  |
Mauvaises ondes | US: Aired Online FR: November 3rd, 2007 |
| 27 (92) | 427 | Cold Sweat |  |
Sueurs froides | US: Aired Online FR: November 3rd, 2007 |
| 28 (93) | 428 | Down to Earth |  |
Retour | US: Aired Online FR: November 3rd, 2007 |
| 29 (94) | 429 | Fight to the Finish |  |
Contre-attaque | US: Aired Online FR: November 10th, 2007 |
| 30 (95) | 430 | Echoes |  |
Souvenirs | US: Aired Online FR: November 10th, 2007 |