From Paidcontent.org:

“We have a $15 billion market cap. If I take Publicis Groupe, which is roughly half that, and IPG which is a third of that, its a combined $50 billion market cap. And all those companies’ revenues put together is $33 billion. In comparison, Google’s revenues are two-thirds of the top four ad holding companies and its market cap is more than four times those same companies.”

To get a sense of the power the Big Four wield, check out this post on how the major ad agencies command nearly 60% market share:

That being said, if you could invest in a basket/portfolio of the Big Four vs. Google, what would you invest in?  I don’t doubt the revenues of the Big Four being so robust, but what about costs, growth, profits?