When I was a MS4, there was one I received via e-mail, and I called within 15 min of getting the e-mail. Of the three dates available, only one date was left, and the spots were nearly gone. In addition, I was taking Step 2 CK when I got another e-mail, and I called within 45 min of getting the e-mail. I got the last spot available!
With smartphones being commonplace, it's the norm to have interview slots fill up within 30 min of the invites going out, so you have to be ready to call as soon as you get it. It's not fun and sucks, but that's how it is now for competitive specialties. Make sure your attendings know beforehand at the first day of the rotation so they know what's up; most will be understanding since they know that if you're a MS4, you're either taking that rotation voluntarily or doing it as something to graduate.