WARNING: The following contains spoilers for Season 4, Episode 8 of Attack on Titan, "Assassin's Bullet," now streaming on Crunchyroll, Funimation, Amazon Prime and Hulu.
Though Attack on Titan's eighth episode of Season 4 pumps the brakes where full-scale action is concerned, it's quite possibly the most incendiary installment of the anime's concluding season yet. Not only is a key member of the Survey Corps, Sasha, unexpectedly slain by a vengeful Gabi, but the young Warrior candidate's intrusion on the Scouts' airship reveals something equally unexpected: Zeke Jeager is also on board the vessel. Not as a prisoner of war, mind you, but as an ally of Eren, his younger half-brother.
Zeke, along with a few others, is the traitor of Marley that Willy Tybur and Commander Magath suspected of assisting Eren's attack on Liberio. Having been Paradis Island's biggest adversary in Seasons 2 and 3, it goes without seeing this is a monumental heel-face-turn, one that will change the course of the series in unfathomable ways. The biggest question for fans right now, as well as why, is when did Zeke turn on his motherland and join Eren's cause? Though it seemed like an incredibly unremarkable detail at the time, a previous episode in the season actually sheds some light on this.
In Season 4, Episode 4, "From One Hand to Another," Eren -- in disguise as Eren Kruger, an injured war veteran -- gives Falco Grice a letter addressed to his "family." We know now that he used these letters as a means to communicate his plans with the rest of the Scouts, who were forced to then come and back him up when he laid siege to Liberio. However, Zeke is also Eren's actual family. It's possible he could have also been covertly mailing these same letters to the War Chief internally, too. Possible... but dangerous. After all, leaving a paper trail would be incredibly risky, especially as Zeke was aware that he and his Warrior Unit were being bugged by the military.
It's far more likely the brothers met in person, and Episode 4 actually provides solid evidence of this. Later in the episode, Falco talks to Eren on the grounds of the hospital he is staying at. Eren thanks the boy for all his help sending the letters, which is when Falco notes a catcher's mitt next to Eren and asks if his family sent it to him. Eren affirms this explanation, admitting that though it's a nice gesture to try and keep him entertained, he can't do much with it given his current condition. Knowing now that Eren and Zeke are working toward a common goal, it's hugely significant that an item so closely tied to the elder Jeager brother just happened to be right next to Eren at that point.
We know Zeke has an affinity for baseball because of his devastating pitching ability as the Beast Titan. There's also no reason the Survey Corps would send Eren something that he had no use for, and especially an item connected to a sport that, as far as we know, doesn't exist on the Island. (Unless the Islanders have spent the past four years upping their cultural and leisure activities.) The only logical explanation is that the mitt came from Zeke himself, which means he must have paid Eren a visit at the hospital.
Could it have happened on the very same day that Falco visited him in Episode 4? Did the estranged brothers play a game of catch and get to know each other a little better? (Eren did also have an actual baseball.) If that's the case, Eren would have needed to be in much better physical shape, so it may be they were meeting up prior to Season 4's beginning, and Eren mutilated himself as their plan began to take shape so he could get himself into a more strategic position.
Then again, why would the mitt be right there at the time of Falco's visit? Both possibilities may be true: Eren and Zeke's collusion could go back further than we think and he visited Eren on the day of the festival to put the final preparations of their attack into place, which means the last letter Eren had Falco send to the Scouts was probably to let them know that Zeke's death needed to be staged so he could come back to the Island with them. Attack on Titan is nothing but masterful in its foreshadowing, and the baseball mitt is an excellent example.
With Eren in possession of the Founding Titan and Zeke in possession of the genetic requirements to use it, both Paradis Island and Marley have a lot to fear from a Jeager brother team-up.