regulations, regulations...
the planning your 4th year schedule depends on several factors. if you think you have a pretty secure shot at matching, then not to worry.
if you have some concerns about not matching, then i think it would be worthwhile to do as many visiting ortho rotations as possible. spend the first month studying "ortho" during the required FP rotation, then do ortho rotations until the interviews. remember though, you are going to be asked ANATOMY questions, not the fine details of operative orthopaedics. know your anatomy INSIDE AND OUT.
if you haven't done the 3rd year surgery rotation yet, i recommend that you rock that rotation and get a good letter. this will free up your 4th year schedule for another ortho month.
in selecting 4th year rotations, keep in mind many programs select heavily from their visiting student pool (for example: UTSW, temple, mt sinai NYC, san francisco program... look up previous posts). there is a strong regional bias in the selection process as well. also, some programs will let you rotate for 1-3 weeks. in all honesty, there is no difference between a 3 wk vs 4 wk rotation, and you could squeeze in an extra rotation here and there if you play it smart.