I'm going to watch replies to this with interest, as I honestly don't know the answer to this one! The 8000 was developed by the service department rather than the design department so I wasn't really involved (we were busy designing what became the 9000 at the time).
The feed through the two main stereo paths should sound identical to the 4000, but if you use the audio sub groups I don't remember what the 8 bus sub-mixer design was like - although I have a strange feeling I may have designed it! (From vague memory - we are talking 35 years ago! - I think we developed it for an earlier project, and then it was incorporated into the 8k.)
As you say, the channel strips are basically identical bar the routing switching.
Cheers,
Andy