I heard of someone from the LA area who only applied to 3 residencies (Stein, Doheny, and UCI) because her huspand was a surgical resident in the area. She didn't match and ended up matching into an EMed residency at Harbor-UCLA. During her intern year a PGY-2 spot at UCI opened up, they called her because they liked her and knew she didn't match, and now she is a resident there. So it all worked out in the end for her.
I would only apply to 4 programs if you have constraints as to where you can live (e.g. significant other, sick relatives, etc.) and if you are willing to apply for another field at the same time. Also, if you want to stay at your home institution and they tell you that they want to keep you, then you might be able to get away with only 4 applications. Otherwise, the best advice is to apply widely. Who knows, you might end up liking a different program on the other side of the country and end up matching there.