I wouldn't sweat it right now... anyone who you'ld be interested in requesting a letter from sure as heck better remember you in six months. But, I think you're right to be thinking about how you could pull a letter or 2 from your MS3 rotations. Others may disagree, but, I think a letter from one of your medicine attendings makes a good 4th (non-ortho) letter. It seems that the med attendings write really good letters, have gotten to know you pretty well, and can add an element of breadth to your app. But, hey, my n=1, and all I know is that it worked well for me.
Good luck... and you have my empathy... getting letters together (and people to get them in on time) can be the suckiest part of the app process, because it's one of the few aspects of it where you are totally dependent on others.

Definitely don't stress about it too much, but you should be asking as many people as possible for letters. Worst that can happen is that they never write it, but if they do, you just have more options to send in. I think I ended up with 3 or 4 letters from 3rd year and used 1 of them. If you can, try and get letters from the program directors for whatever rotation you're on. Even if it isn't ortho, if they say things in your letter like they would love to have you in their program if you changed your mind about ortho, that can surely help out.