Shueisha’s first printing of One Piece Vol. 69, which shipped on Monday, is four million copies, according to Mantan Web via Anime News Network.  Initial print runs have been around 4 million since Vol. 65 in 2011. 

Print runs were at 3.2 million with Vol. 59 (see "The Most Popular Manga in the World"), so the title has grown significantly in the last few years.  One Piece creator Eichiro Oda was making $24 million a year in 2010 (see "Manga Creator Makes $24 Million").
The series is in the middle of the pack of the Top 25 Manga Properties in the U.S. (see "Top 25 Manga Properties").