I think I'm personally off the iZotope upgrade roller coaster. I have MPS 2, but I also got the Tonal Balance Bundle a bit ago so I have Ozone 9 Advanced and Neutron 3 Advanced. I don't think MPS4 would give me anything I'm particularly interested in - Nimbus and R4 would be nice, but that looks like that's about it. I don't really care about NeoVerb, and Nectar 3 Plus doesn't seem like a significant change from Nectar 3 which I already have. I guess it'd also give me RX 8 vs. RX 7.
When Ozone 10, Neutron 4, or Nectar 4 drops, I'll wait a while and see what discounted offers might be available, but I expect that to be a year or more.