you might burn out- which is bad.....

If you feel that your credentials aren't as competitive as they could be, and you have an easy-going personality that will enable you to endear yourself to the residents and attendings, then go for it. Following your rotation you will be almost assured of getting an interview at these programs if you perform well.

I agree. I did two rotations at my home school and one away (all one month long). Another way of bypassing the burnout problem is doing a two week rotation. I think most residents/attendings have made their mind about a student within the first two weeks. It would be rare to find a student that strong to end weak. If I were to do it all over again, I would have done 2-3 away two week rotations in addition to my home school.

However, don't you guys think that you would not be as high up in a program's list if you did a 2 week elective versus a 4 week one? Especially, if you're doing 4 week electives at other places? Or, does that not matter?

That's still 2 weeks more than many, many other applicants will have done at some programs. I think that 2 weeks is looked upon favorably versus having not rotated. 2 weeks versus 4 weeks, tough call. But I think that if you can get a couple residents on your side (who can talk you up in ears of the powers that be) in just a few your fine. Be honest to yourself and ask how good of an impression do you think you can make with limited time. If you take a little time to get going, do the 4. Either way, a rotation is only a small portion of a successful match. A way to get your foot in the door. You still must show at the interview and make your interest very well known.
I agree with what has been stated earlier about doing 4 rotations if you feel like you're trying to compensate for other holes in your app, otherwise I would consider scaling it down a few. It's much easier to look bad on an away than it is to look good.