The latest patch is supposed to reduce the RAM utilization. May be interesting to check
v1.1+193 against an older build regarding encodability of 4K video even with a 32-bit build.
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Stable at 1.6 GB with preset "slow", looks promising.
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Preset "slower" requires about 1.9 GB RAM (Private Bytes: 2,040,092 KB / Working Set: 1,768,604 KB – at frame 30); the patch must have made it more dependent on the complexity, instead of assuming a maximum always.
I believe there may still be headroom with more efforts, but a further reduction will probably be harder to achieve (not without some deep reorganization and allocation "smartness"). For now, even on almost "obsolete" 32-bit OS, 4K encoding will be possible with preset "slow", maybe even "slower".