The 4100 is not a bad sounding amp, far from it. It is as good as any Marantz that I've owned included the legendary 2285 (non-B), which would be the closest competitor. The problem is the cost which IIRC was $1600. It's difficult to convey how cheap looking and feeling the top is, you have to see one in person.
Yes they were $2G when they were last made in 85. In my case honestly I've never paid attention or been bothered by the vinyl clad case on my 4100. But then again I also overlooked the vinyl clad cases on my Sansui G series as well because they were good performers.