4 I didn't have your email address, so I couldn't contact you. If (1/have) your email address, (1/send) you an email.

4. I didn't have your email address, so I couldn't contact you. If I had had your email address, I would have sent you an email.

В данном предложении использовано условное наклонение третьего типа. Первая часть сложного предложения (If I had had your email address) выражает нереальное условие в прошлом, а вторая часть (I would have sent you an email) - результат, который мог бы произойти в прошлом, если бы условие было выполнено.

Ответ: I had had, I would have sent