"Come back to us, you hear? Come back to us."

Fours Grandma to Four in a flashback. He never did go back.

Four's Grandmother, along with her husband, raised Number Four when he was young before the Mogadorian Invasion of Lorien. While her husband had a joking and carefree personality, Four's Grandmother was more quiet and reserved and very intelligent. 

On the day of the Mogadorian Invasion, Four's Grandmother was frantic and sad and cried when she had to say goodbye to her Grandson, telling him to come back to them one day. Once Four had departed she would have entered the battle and lost her life fighting the Mogadorians.