Looks like Liberty bellows had one for sale last year for about 2500$ but keep in mind this was sold with a warranty from an accordion shop with a decent return policy. I am always a bit suspicious when I hear a claim about the condition of a 50 year old accordion. To me excellent condition means the unit performs as it was designed to, with no degradation of performance from absence of appropriate maintenance.
I've never seen or read about one... the designation for Continentals was always 5/6 as far as I know. I've only seen maybe 3 (2 Continentals and 1 Grand), but seen maybe 200-250 articles/pages/sales over the years, never saw a 4/5 to 4/6 model of this designation.