Many runners have now run one mile under four minutes. The first to accomplish the feat was Roger Bannister in 1954, with a time of 3 minutes, 59.4 seconds. Since this time, the record has been broken many times.
This link has the history of when the fastest mile time was broken:
http://www.infoplease.com/ipsa/A0112924.html
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A friend of mine was the 1000th person to run a 4-minute mile about 5 years ago, so I would estimate the answer is about 1000-1100 people.
According to the Track and Field News archive, as of October 1, 2013, there have been 1303 people who have run faster than 4 minutes in the mile.