Hello! I was wondering if a quad relationship in which all partners are together, romantically and sexually, is possible?

Yes, it totally is!

I don’t have an FAQ specifically for this type of “is [XYZ situation] possible/a thing” question (yet), but here are two recent posts of mine that have answers nearly identical to the one I’d give you:

Two couples forming a quad

Have I ever heard of childhood friends creating a polyfidelitous relationship?