The version 4 is a dead end box and is not feasible to upgrade further nor is upgrading it supported. Your only upgrade path would be to purchase an 861v8 and sell off the v4.

That said, there are many advancements from the v4 to v8 that are well documented here in the forum. If your system is all DSP speakers, a v8 will have some connectivty challenges as it is all SpeakerLink. If analog, that is no issue.

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One step from the Presidency
#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