Should I have a separate library for 4K movies?

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Depends on your users. If you have remote users streaming remuxes, I would put them in a separate library. That goes for clients that require transcoding too, as that can get out of hand quickly. So, in general, a separate library is the safest bet. The caveat: if it's just you and/or your household and you have ample transcoding and tonemapping abilities on your server, go wild.