They even had a 6-Star; in 1976, for the US Bicentennial, the US Senate voted to elevate George Washington (posthumously, of course!) to 6-Star rank. Can't remember what is was called, though.
No, not quite. They posthumously promoted Washington to the rank of 'General of the Armies' (closest thing to a title we have), but stipulated that he would come first, in order of precedence, over all other 'Generals of the Armies' - I believe Pershing was the only other one, but he never wore more than 4-stars. Washington's 'promotion' didn't actually have any new rank, rather it was more titular...
The 5-star rank is 'General of the Army' singular...except for Hap Arnold, who was redesignated 'General of the Air Force' after we split off as a separate service