Thanks for the info DANCAD3D. My first response is that I do not expect to add complex filtration and then play back 4K at full frame rate immediately, I would be happy with a single frame for immediate preview and then RAM cache rendering in the background.
In any case, my primary DI objective is to play back 4K material in real time, even if that means that image processing steps like color correction have been rendered out in advance. Ideally some sort of large RAM cache would allow me to make changes per the client request and with a 10 second wait I could play back at least a few seconds from the RAM cache at close to full quality.
My quest is for a beefy desktop solution that gets me 70% of what I can get with "big iron" at a reasonable price point. I am cool with days or even a couple weeks of rendering after sign off for a 90 minute feature.
More important to my pipeline is 4K dailies to check focus, noise levels, how the makeup will look at high resolution, application of a print LUT if film out is in the cards, to check lighting ratios, etc.