A bit less than one year ago, on April 23rd 2009, Apple reported 1 billion apps downloaded.
158 days later, on September 28th 2009, Apple reported 2 billion apps downloaded.
99 days later, on January 5th 2010, Apple reported 3 billion apps downloaded.
93 days later, on April 8th 2010, Apple reported 4 billion apps downloaded.
The download rate for the past 93 days was 10.7 million apps per day.
Given 85 million iPhones and iPod touches sold, the 4 billion app downloads amount to 47 per device. (Although that number is likely to be considerably higher due to many iPhones/iPods having gone out of use–an attach rate of well over 50 seems like a good estimate).