The four who have left include design lead Jeff McGann, principal systems designer Kenneth Strickland, director of product development Tim Gerritsen and art director Nate Wells. Wells' departure is particularly noteworthy, as he has been with Irrational for nearly 13 years, having joined the company in September of 1999.
Gamasutra reports that Wells' Twitter bio briefly mentioned he was changing jobs, though the reference has since been removed. Furthermore, his LinkedIn page noted developer Naughty Dog as an employer in 2012, though that has also been removed.
At the very least, we know who's replacing Wells on the project, namely Scott Sinclair, art director for the original BioShock. Ken Levine himself confirmed as much on Twitter, stating that Sinclair is "back in the art director's chair for Infinite to bring it home." He added that he "Can't wait" to show fans "what's cooking." The wait could still be a while, as Levine has made it clear that we won't be seeing Infinite again until it's practically finished.
Meanwhile, Joe Fielder, formerly a producer at Irrational, is now listed as a writer.
It's worth noting that BioShock Infinite is nearing its February 2013 release date, and it's not uncommon for studios to shed staff as development draws to a close. Joystiq has contacted Irrational for comment.