The fourth year structure varies by school. It is true you can choose and apply to where you want to go ..... they can be all in the same city to all over the US as said before. The fourth year externships are also a way to get a look at residencies your interested in ( for example som residencies will only consider applicants whom have externed there and other) and to get some great experience in a specific area ... its all based on your motivation and whether or not your willing to travel and can afford it ...... for example the school im at you have 5 months where you extern at wherever you want with one specifically private practice and two specifically hospital based, the other 6 months are spent at the school and surrounding clinics and hospitals as assigned ..... i hope this helps ..