I agree to a point that no, 3 vs 4 weeks won't make a difference. Getting a dept. chair letter means that you 1) will have worked with him/her one on one and he/she knows you well enough to write a letter, or 2) you have made a good impression on an attending or two and they recommend you through comments in an evaluation. In my away elective experience, dept. chair contact was pretty limited. I ended up asking an attending that I worked with for 2 weeks. I think that a mediocre dept. chair letter that says 'so and so was here for 3 weeks, showed up on time and was interested' is worse than an attending letter saying 'you were great, a real standout, very interested and I have observed these qualities firsthand.' There are a few variables, however. The main one being exactly who the chair is and where they are at. If it's a great nationally known program, then any letter might be good. Where would you lose this week anyways if you did 4 weeks? If it would throw off the rest of your year then forget about it, but if you lose a week of vacation then think about doing 4 weeks and chalking it up to an investment in your future in getting a great over a mediocre letter.
