You deposited $1,000 in an account earning 8% annual interest at the end of 2013. After 2 years (i.e., at the end of 2015), your dad made another deposit into the same account. At the end of 2017 (i.e., 4 years after your original $1,000 deposit), the account balance was $4000. What was the amount of money that your dad deposited into your account at the end of 2015?