Zack received a gift of stock from his uncle two years ago. Zack's uncle had a basis in the stock of $4,000, but the fair market value of the stock on the date it was given to Zack was only $1,500. Zack held the stock until this year and just sold it for $4,200. What basis in the stock will Zack use to determine his gain or loss on the sale of the stock?