What are the four cheeses generally used in"four-cheese" pizzas? In common consumer pizza, microwave and bake-at-home pizzas are often advertised as four-cheese pizzas. What are these four cheeses, traditionally? Do different brands use different mixes, or does everyone stay about the same? How are these cheeses traditionally mixed, in what fashion?
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