What are the 4 leading game producers in the united states?

I need to know what are the 4 leading game producers in the united states for my economics project. If you could please just list off the 4 or more form largest or best to smallest or worst. THANKS! Also if you could provide me with links to back-up your statement that will get you best answer :) THANKS AGAIN EVERYONE!What are the 4 leading game producers in the united states?I know 3 of them are Activision, Ubisoft, and EA.