He ... to ride a bicycle when he was four years old 1) wasn not able 2) cannot 3) shiuld not 4) could not

He could not ride a bicycle when he was four years old.

He could ride a bicycle when he was four years old.

Could he ride a bicycle when he was four years old?

Глагол can имеет форму прошедшего времени could, которая переводится, как "мог".

Could используется для выражения возможности что-то сделать в прошлом.