What are the four cheeses generally used in"four-cheese" pizzas?
They all seem to use different cheeses depending on the brand. 


The most consistent cheese is definitely mozzarella, which makes sense as it's the default cheese on most pizzas. 
Second to that is some sort of hard grating cheese like Parmesan or Asiago... which also makes sense as it's a traditional pizza condiment.
Other Italian cheeses also seem popular like fontina and provolone
But non-Italian cheeses like cheddar certainly show up.


Below are some examples. I don't recommend any of these products, they just came up in web searches and specifically describe which four cheeses they use.

Amy's uses: Fontina, provolone, Parmesan, mozzarella:



Frontera uses: Mozzarella, white cheddar, smoked provolone, asiago.



California Pizza Kitchen uses: Mozzarella (apparently more than one kind), fontina, gouda



Lean Cuisine uses: low fat mozzarella, Parmesan, asiago, romano