Lucky Number 4?

The other day I was sitting in my class and we were discussing how the female role is portrayed nowadays in the media. I found it really interesting that a lot of movies and television shows that were released recently have 4 main females. Sisterhood of the Traveling Pants, Sex in the City and Desperate Housewives are just three examples of shows where their are 4 main female leads. I think the most interesting part is that each female expresses a different "type" or women so to speak. I think nowadays women are seen as a more dominant and independent force. Whereas, in the past they were overshadowed by the male actor. I like that these women are the main characters but I'm not sure if they really provoke the right idea? You always have a "dumb blonde", an unattractive "geek," the quiet one, etc. Does every blonde actress have to fall under the "dumb blonde" category? I don't know maybe I'm being to picky but I was just intrigued to see how these main roles usually come in 4's and each character has very different characteristics.