Here is the answer from MadVR with regard to 4:4:4 with each of their video processors:
Core MK2 without Premium Pack — 4:4:4 input?
Yes, it can — with an important nuance around frame rate. The Core MK2 without the Premium Pack operates with HDMI 2.0 input (18 Gbps), which is sufficient for 4K24 and 4K30 4:4:4 12-bit. Where it falls short is 4K60 4:4:4 12-bit, which requires the higher bandwidth of HDMI 2.1 — and that requires the Premium Pack on the Core MK2. So the answer is yes for 4K24/30 content, and no for 4K60 4:4:4.
All versions of the Extreme — 4:4:4 input?
Here it depends on which generation. The Extreme MK1 and MK2 have HDMI 2.0 input, so they share the same limitation as the base Core MK2: 4K24/30 4:4:4 is fine, but 4K60 4:4:4 is not. The Extreme MK3 has native HDMI 2.1 input (48 Gbps), so it handles 4K60 4:4:4 12-bit without any restriction.
For your Strato K installation specifically:
The good news is that the Strato K's 4K Cinematic content — including its native 4:4:4 titles — is 4K24. That fits comfortably within HDMI 2.0's 18 Gbps, which means every Envy model, including a base Core MK2 without the Premium Pack, will accept and preserve the full 4:4:4 signal from the Strato K with no issues whatsoever.
On maintaining 4:4:4 through the entire chain:
The Envy passes through whatever chroma subsampling it receives — it does not alter it. So if the Strato K sends 4:4:4, the Envy receives 4:4:4 and processes it as such. (The Envy's chroma reconstruction from 4:2:2 to 4:4:4 only applies to content that isn't already 4:4:4 — native 4:4:4 content needs no reconstruction.) The key variable for maintaining 4:4:4 end-to-end is ensuring your source device settings and cable quality support it consistently — the Envy itself is not the limiting factor for the Strato K's signal.