4. Jack and his friend/not / to ride bikes/for three hours.

Решение:

• 4. Jack and his friend have not been riding bikes for three hours. (Отрицательная форма Present Perfect Continuous. 'Jack and his friend' — множественное число, поэтому используем 'have not'. Действие длилось 'for three hours' и, возможно, продолжается.)

Ответ: Jack and his friend have not been riding