The "Quad" is a military alliance defined as "The United States, Japan, Australia and India"

  the United States, Japan, Australia and India first resuscitated their informal dialogue from a decade-long hiatus in late 2017.

It is  informal but growing stronger every day because fo threats from Russia and China alliance between the U.S. and Japan and Australia and India.

This is becoming more of a Military alliance that over time might be as strong as NATO in the North Atlantic with the Quad being a Pacific Alliance instead. And it is also possible that other nations like Thailand and Viet Nam and others might join this alliance eventually too. If things get worse you might also see the Philippines and Indonesia and other countries join the "Quad" too but it might be given another name if that happens in the future also as more countries join this alliance.