I have an AC version of the 4002 and it is a lovely deck. I was looking for a 4000 as it was the first in the series and had some of the overengineering that tends to get me quite excited, but as I couldn't find one I settled on a very nice 4002 that I bought from eBay.
Peter is the expert here and thinks they sound similar, so I guess the questions is whether you'd prefer the first of the line with its eccentricities, or the slightly simpler later models that are perhaps easier to work on etc.
Of course, if you really value rarity and eccentricity in equal measure, why not go for a Beogram 6000 with the CD4 thingy in it just because you can!
John
President, Beomaster 8000 Appreciation Society