It's hard to say, since our program is still relatively new. The first class graduated in 2009, so even if they did a three year residency like IM, they would have just finished two months ago and they'd be first year fellows now. In other specialties like surgery with longer residencies, they'd be fourth year residents now. We'll have to wait another 3-5 years before we can see what kind of careers the first few classes of CCLCM grads have.
As for what to say about why not a fifth year, you should just say what you said here: that you're interested in research, but you're not sure you want a research based career. There is an option to do a fifth year for research through the UP, so you can always do one later if you change your mind. To me it sounds like what you're trying to say is that you want an academic medicine career. That would give you the option to pursue research without it necessarily being a requirement, assuming you got hired on a clinical medicine (i.e., practice) track as opposed to a research track.