Well, Yahoo! points out that Google's market cap isn't four times more in value than Yahoo!'s. Google's market cap, Yahoo! says, is, for the moment, about $4 billion more. Google's hovers around $51 billion, while Yahoo!'s is about $47 billion. Of course these figures will fluctuate. Google's share price, Yahoo! says, is four times more, but when you calculate total shares outstanding, the market value is not four times more. Is that clear?
Basically, while Google's share price is four times more than Yahoo!'s, its market value is just $4 billion more, for the moment.