The anime about a 4-year-old is likely Kotaro Lives Alone (Japanese: コタローは1人暮らし, Hepburn: Kotarō wa Hitori Gurashi).
Kotaro Lives Alone is a Japanese manga series written and illustrated by Mami Tsumura. It focuses on Kotaro Sato, a peculiar four-year-old boy who moves into an apartment building all by himself. He's mature beyond his years and often says things that surprise the adults around him. The story explores themes of loneliness, family, and found connections. Kotaro's background is shrouded in mystery, and as the series progresses, glimpses into his past are revealed, adding emotional depth to the narrative.
The anime adaptation is available on Netflix.
