The difference is hardware. A hardware version 4 can run version 8.0 or 8.1 firmware. Although Meridian did not support upgrading a version 4 to 6 or 8, presumably due to the age of the backplane and other components, it is possible with the correct cards on hand to make a version 4 a 6 or 8. I have personally made a version 4 a version 6 due to the lack of available OE12 cards.

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#1 Trinnov Altitude 32-8-16, 271, 8000XE, 7200SEHC, 7200.2, Seaton F18, JL Fathom F113, Revel w990's, Kef R7, ATI 528 Amp, JVC RS4100K, SI screen. Roon Nucleus+, Eno AG, Eno M link, 218.
#2 Mark Levinson No5105, Ayre VX8 and CX8, Vandersteen Treo CT, Preamp varies.
#3 Meridian Ellipse