i have not but plan on doing one 4th year and have had friends who've done them. they have all had tremendously good experiences. the way that it is set up now (i think, someone can correct me if i'm wrong) is that they will provide you with the stipend but you have to find your own externship clinic/location/program. i believe they should have a list of externship sites where people have worked. you can find their email address at www.ms4c.org sorry if any of this info is redundant!
and also, if anyone else is interested, you don't have to do it during your 4th year. many have taken a month during the summer between 1st and 2nd year. because there's such a strong emphasis on education, it's a very hands-on experience regardless of where you are in med school.