You're misunderstanding a little -- he's saying that the USA is like its own Schengen Area (according to wikipedia, ~400 million). So for travelers from outside the EU, you just need one visa to enter and then you can move about freely. Similarly, you just need one US visa and you can go from NY to California freely.
I probably (wrongly) conflated the two parts (4 states are larger than.. vs. large area in which you can move rather freely). No offense intended and I see the point now.
