Remember, it’s “marshal” with one L. The story goes that, after the U.S. entered World War II and its military was drastically expanded, FDR didn’t ask Congress to create the rank of field marshal in keeping with the British practice because he didn’t want to have a “Marshal [George C.] Marshall.”
David McCullough told of President Truman, at the Potsdam Conference in 1945, being amused by the Soviet delegation’s deference to a U.S. marshal who was part of his entourage.
The “proper” form in the United States is just plain “general,” it originated historically as “captain-general,” but that is a historic form only. To the extent that there might be ambiguity, you can say “four-star general,” but it’s not proper or formal, just like “full colonel.”