I wouldn't be too worried about them, For the most part they are great and reliable transformer. There are sooo many of them out there that finding a good used Z4000 is pretty easy to do anymore. Much like the Postwar ZW, these things will be around at Shows and Auctions for a very long time, and when treated properly they should last a lifetime or more. Much unlike the postwar ZW, they are not as simple to repair yourself and sometimes not repairable at all.
My dad has a 1998 model that was part of the first production run that he bought new. Excellent transformer used a lot in the last 22 years and works perfectly. Tried a ZW-L when they first came out but didn't like how the chopped sine wave made the postwar engine buzz and growl, so it went down the road and the Z4000 stayed.
