From what I've seen there are a couple of different factors. Only an MS4 though. One thing to remember is that it isn't about how open and welcoming an institution is to couples matching, but about finding a place where BOTH individuals will match, as individuals. I'm pretty sure that made no sense by the way.
Ultimately it's all down to competitiveness.
I met quite a few people on the psych interview trail that had ended up at 6th, 10th, and even lower choice because of couples match. Because their spouse/SO was interested in a competitive specialty but wasn't necessarily the most competitive applicant (or even if they were), the spouse's specialty choice ended up being the crucial factor in determining where they ended up.
I hope that makes sense. Basically how easy the couples match will be and how high on yalls list you end up is determined by the person entering the more difficult specialty and/or which person is less competitive within the specialty they're applying to.
On the other hand, I've certainly heard of (gotta love rumor) cases in which a spouse/SO rides on the coattails of a stellar applicant that a strong and influential department absolutely wants.
For psych and IM, things should go pretty well since with both people tend to get one of their top few picks. In addition, there are many cities in which there are multiple programs in each specialty, which also makes things easier.