-Takeshi has to go up and down various staircases. However, he finds one he must go down to is flooded. She tells him to swim. Takeshi speaks in disbelief, asking if a hamster can really be there. Tsugumi explains he must have used the air ducts. Takeshi approaches the water, and hesistates for a moment.
-Tsugumi tells him that's enough, and she'll head over there. Takeshi says no, he's just getting prepared, and Tsugumi tells him he'll have to swim to a staircase that's 51 meters (167 feet- Hirameki got it wrong when they said it was 153 feet) away, filled with cold, flooded water. Takeshi thinks if he can make it.
-Tsugumi asks if he's okay, and he replies, "Oh, so you're worried about me?" She angrily replies no. Takeshi tells her the farthest he's swam was 40 (131 feet) meters in high school, so 11 (37) more should be fine. Tsugumi says, "Wait! I really... should go." Takeshi replied, "You're that worried about me?" Tsugumi: "Don't misunderstand me!" Takeshi then says he'll dive, and does so.
-Takeshi starts really losing breath at 30 meters, but urges himself to keep going. At 40, he feels like he's going to pass out, and his eyes close. He remembers everyone's faces in order of Kaburaki, Coco, You, and Sora (he also mentions Pipi), and then Tsugumi. He imagines them all sad and full of despair. He tells himself he was going to make Tsugumi realize there's hope in the world, and he opens his eyes and keeps going.
-He finally makes it. After catching his breath, he hears Tsugumi yelling into the earphone, "Hey! Can you here me!? Give me a reply!" When he calls her name, she asks if he's all right, and says yes. However, she tells him to take a short break, and he agrees.
-After his break, he enters a warehouse, where the signal is coming from. He calls out, "Hey, Hamster!" Tsugumi says not to call him that, call him by his name. He asks what its name is, and she quietly says it's "Chami". When Takeshi goes "huh?", she shouts that his name is Chami, which hurts Takeshi's ears.
-He calls out Chami, who appears. Takeshi grabs him, and Tsugumi expresses relief that he's okay. Takeshi places Chami in a plastic bag, and goes back the way he came. He gets through easier this time, knowing he's responsible for more than just his own life this time.
-When he emerges, he sees Tsugumi is waiting for them with a concerned face. Takeshi gives her back Chami, and we see the Chami-Tsugumi CG. Takeshi just stares, overwhelmed by how happy she looks. She then blushes, asking what he's staring for. He says "no reason", and she goes back to looking at Chami.
-On their way back, Takeshi think he should go back to the hot spring one more time. Tsugumi says "Thank you..." and Takeshi just says "You're welcome."
-Afterwards, Takeshi goes to bed.
-End Day 4. 55 hours remain.