I've only used the 8000 but I think that they're all basically the same inside. Cold forged aluminum alloy main gear with some black hardening coating, and a bronze pinion gear. Real simple internals. Easy to take apart and put back together.
I've used a 4000 Sustain on my 7 footers, with braid. They seem like a good match. 4000 model Shimanos all weigh about the same and have the same capacity, I think.
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