I'll say 4 here, as it's really more of a 3.5 for me. It's an important quality, but it's just short of essential in what I like in my partner (for those keeping track, the essential parts are a minimum of three shared interests outside the bedroom, a sense of humor, and a willingness to bang only me in terms of having guys involved) but it's just in that next level of importance.
As Peter said, though, that doesn't mean I want a chick that can tell me what the 32nd digit in pi is without hesitation or recall what every policy put forth on the ballot means, but if she can talk to me that displays an ability to string sentences together that forms an idea or clearly describes her position on the topic at hand, it's a major turn-on. I can only deal with an absolutely hot woman looking at me dumbfounded over every possible source of conversation I offer before I want that face to actually respond.