Tegami (E04)

Rin befriends Kouki, a boy she met at the local nursery school, while Daikichi's pulse races a bit when he meets Kouki's beautiful single mother. Later, the modem Daikichi discovered at Souichi's house leads him to his grandfather's last words. And from there, he receives a contact number for Rin's mother, Masako... (Crunchyroll)