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Netflix probably want delivery in 4K RAW (if possible) so that they are 'future-proof'. At least that is what I read in some trade magazine. With Japan starting 8K transmissions, it will not be long before someone somewhere insisting on material being stored and delivered in 8K RAW. That is 16 times the data for HD!
The reality is of course that nearly all home viewing is done on conventional flat TV screens and those are not very large. They have their own compression algorithms that adjust to the speed of the local network of the customer, so the end-user can get just SD or below SD quality.
The reality is that you need the customer to be watching on at least a 50" TV set, before they notice the difference between good SD and good HD. This issue is clouded by the problems created by so-called 'lossy' compression. Some terrestrial feeds that claim to have HD quality are below the resolution of a good quality SD. The punter is only watching on a small TV set and usually is hardly aware of the difference!
The main cause of this difference between good and grotty is the number of channels that broadcasters cram onto one transponder (satellite and terrestrial TV) and the amount of compression used to get an entire movie, plus special features, the making of and a series of pointless ads for movies I don't want to watch, onto one Blu-Ray disk or DVD.
So the carefully crafted image made by the DoP and the people in finishing and mastering can be totally ruined (digital artifacts, blotches and colour aberrations) by commercial considerations being made about additional material to be stuffed onto a disk or additional channels on a transponder.
But it gets worse!
If you are watching a movie via a broadcaster (regardless of medium) the audio will be mashed and ruined by so-called 'look-ahead' compression that reduces all loud noises and pumps up the quiet passages. Gun and cannon fire sounds feeble and quiet whispers are brought up to the same level as an atomic explosion.
So (IMO) unless you know that your end-user will be watching on a 4K projector for a 2-3 meter screen and with a proper 5.1 or 7.1 sound system, I would take all this talk of things having to be delivered in 4K with a pinch of salt.
But all that hardly helps you if the customer wants and asks for delivery in 4K RAW. If your present iMac does not cut it, it might be worthwhile looking at a more powerful PC or just doing the editing etc. off-line and creating an EDL to be rendered later.