I'll throw in my 2cents and advocate for 3 or 4 EM rotations. I was lucky enough to have a home(ish) program where I did my first EM rotation. I decided pretty early on that I wanted to do 3 aways for a number of reasons:
1. Get a flavor of different styles of programs. My home program is more of a county ED, so I wanted to compare it to a community, tertiary, and super academic ED. Further, each ED is run differently (ie pods vs grab a chart and go vs sections). Finally, it is interesting to see how lectures are so different in each program.
2. Diversity for region. I think this helped immensely as I did an away on the west coast, S, and SE regions. If you can show you can work with different types of personalities (both patients and staff), I think this will look "good" to programs that you can fit seamlessly into any team.
3. More SLOEs to choose from. If you have a bad experience on one away, don't upload that SLOE. Most of the time your 4th SLOE won't be uploaded until November or December, and by that point you've had a number of interviews as it is. I felt a lot of programs didn't even realize I had done a 4th away as that 4th SLOE wasn't uploaded yet.
4. Overall experience. You learn a lot about EM by getting a ton of different opinions. Some places use TIMI, others use HEART. Some places have ECMO, others hypothermia. Trauma bays are all different. Do you mind hallway beds, or prefer all private rooms?
I think the big caveat is that you have to be confident that you'll do fine on your aways. Doing this many could backfire, as it may increase your chances of getting that dreaded SLOE that torpedoes your app. On the flip side, it could also give you 3 or 4 bomb SLOEs that boosts a lower step score and average grades.
Getting into a residency is all about playing the game. The crappy part is that you have so many decisions to make. The awesome part is that YOU get to make those decisions, so its in your control.
Good luck future EM docs!