Very early versions of C4000 (typically under SN#1000) had sonic hologram A board which did not use the Pade delay lines. There was a factory upgrade to fix early C4000 bugs for $67. I know BillD sent his in, because I fixed and modded his C4000. SH upgraded to version B was the biggest fix. Upgraded early units have a "C" written in black felt pen in front of the serial number.
The champagne units (and I think some of the early anthrocite units) used 4136 op amps in the phono stage, and were not as quiet as higher serial numbers which had transistor front ends.
Units with 35/50 ms rear channel delay most likely will have the delay ICs burned out. They were fragile chips, and low serial number C4000s had a higher voltage power supply in them to give the rear channel amp higher power. But that higher voltage caused reliability problems. Later versions use a lower voltage power supply board. Anthrocite units (gun metal) usually have working rear delay channels. The champagne units are beautiful. You can read the faceplate, unlike the dark anthrocite units.
There were many changes vs serial number, and I have cataloged all of them. I have not looked at the notes in years, and they are in storage. Any C4000 with MM phono input will have most of the latest fixes.
All versions have a not great correlator design, and the balance control does not work with SH. I redesigned my C4000s to fix all those problems. I'm working on another multi-year project but when finished will resume my C4000 project. The C4000 is by far my favorite preamp. It is the Swiss Army Knife of preamps. My latest changes blink the correlator LED when the gates open and shut. I've also reconfigured the SH unit to cancel the cancellation signals, not just the first arrivals. It gives a very different open sound. That automatic threshold for the correlator is a lie. It's a fixed resistor voltage divider.
The problem with the 4000t is the SH is stuck on wide mode, which does not image as well. And Carver deleted the dynamic range expander and substituted the time lens gimmick. The rear amps are missing, but you can use external amps. The correlator is better, though.