i did my first ortho elective away... i received good feedback. a few residents seemed impressed that i would be willing to my first elective away.
the real advantage was that i got every away that i applied for during that time... at many popular places. since everyone else was trying to do their programs first, then aways later, there are a lot of vacancies.
even after the first or second rotation, people don't expect you to know a whole lot (in my experience at least)... so dont let the idea that you have to do your home program first dictate your entire fourth year schedule.
you dont also have to know everything or impress anyone. its nice, but not necessary. being genuinely interested and hardworking is usually suffice.
i agree with painkilla. do what you want fourth year... its the last time you may have to so for a while.