WARNING: The following contains spoilers for Season 4, Part 1 of Attack on Titan, now streaming on Crunchyroll, Funimation, Hulu and Amazon Prime.
Eren Yaeger has steadily been growing in strength since the first Attack on Titan season. His time in the military and training with ODM gear meant even his regular body was in better shape than most people, and he’s only grown stronger since. But that growth pales in comparison to how far his Attack Titan has come and what it’s capable of now in Season 4.
When Eren first transformed in Season 1, he didn’t have control of his Titan form. In its rampaging body, the Attack Titan was no stronger than a Pure Titan of similar size, and equally as durable. As a result, his Titan's limbs were broken frequently punching Pure Titans, and could become easily overwhelmed by a relatively low number if they swarmed him. Once Eren got control of the Attack Titan however, it was a completely different story. His strength increased several-fold, allowing him to lift a boulder at one point that Mikasa remarked would have been impossible for even a human of proportionate size to do.
Even though Eren successfully controlled the Attack Titan once, his Shifter powers were still far from consistent. In order to mold him into a better weapon for humanity, Hange and other elite members of the Survey Corps teamed up to help him master his newfound power over the course of the next few seasons. They succeeded in identifying the trigger to his transformation -- drawing blood, allowing Eren to summon his Titan form at will. Eren also attempted back-to-back transformations, but after one success, he could only produce a mangled, disfigured form on his second attempt.
Eren’s next few power boosts came not from his training, but as a result of him being involved in situations wherein his method of dealing with Pure Titans -- punching and smashing mindlessly -- didn't work. While facing the Female Titan in the Forest of Giant Trees, he was forced to adopt a more tactical approach to fighting as Annie had more experience as a Shifter and was a better fighter. Those tactics were put to the test when Eren faced off with the Armored Titan. Upon finding out his fists did little to no damage to Reiner, he switched his fighting style once again. Despite being the physically weaker and less durable Titan, Eren’s new style of fighting, which included tackles and holds, would have led to Reiner’s eventual defeat had the Colossal Titan not interfered.
During his experiments with Hange and the Survey Corps, Eren tried several times to replicate the Titan hardening ability Annie had shown during their fight. They believed it could be the answer to plugging the hole in Shiganshina, but Eren didn’t have any luck recreating the crystal. He finally gained the ability to harden, however, after ingesting Rod Reiss’ vial of 'Armor' fluid while trying to protect his fellow Corps members. The effect was instantaneous: he hardened his whole Titan form and protected himself and the rest of the Survey Corps.
With Eren now able to use hardening, the plan to retake Wall Maria became feasible. He continued to train with Hange to increase his stamina and master his new hardening abilities. The results? Eren nailed localizing hardening before the battle of Shiganshina and transformed into fully capable Attack Titans twice, one of which he hardened to seal the hole in the wall. Eren’s localized hardening meant he could also concentrate the crystal at his knuckles to add a lot more force to his punches, preventing him from suffering damage from the force exerted. This new buff meant even Reiner’s Armored Titan could no longer withstand a punch from him.
Four years have passed, in-universe, between Seasons 3 and 4 of Attack on Titan, and from the beginning of the fourth season, Eren is now in complete control over his Titan Shifter abilities. This includes controlling the rate at which his injuries heal; nursing wounds for years on end and regenerating whole limbs in a matter of seconds. During his fight with the War Hammer and Jaw Titans, he shows the same accelerated healing in his Titan form. He also manifests whole new Titan forms three times in one fight -- shocking even other Titan Shifters present. Eren also shows an even greater aptitude for battle, applying all the Titan fighting techniques he's learned so far and uses his opponents against each other in ingenious ways. He defeats the War Hammer Titan and eats its holder, gaining its ability to change the landscape and create structures using hardening. Eren now holds three Titan Shifter powers: the Founding, Attack and War Hammer Titan.
The Attack Titan may have another ability that Eren is yet to unlock. At the end of Season 3, we learned that Eren Kruger -- who passed the Titan to Eren's father, Grisha -- knew about both Mikasa and Armin even though they hadn’t been born yet. Titan Shifters accessing memories from their past holders is a common phenomenon, but foresight may be an ability exclusive to a holder of the Attack Titan. This mysterious new power may have some serious implications for Eren in the future.