I was initially dubious of the "4 million year" quote, but the quick searching I did doesn't turn up anything credible that would contradict it, and did turn up many estimates that concur.

I don't know enough to evaluate his methods, but Mark Pagani at Yale seems to have published a number of papers that give estimates going back 40 million years: http://people.earth.yale.edu/cenozoic-evolution-carbon-dioxi...

And this paper 2005 paper by Dana Royer provides an overview of a few hundred estimates going back 500 million years: http://droyer.web.wesleyan.edu/PhanCO2(GCA).pdf