CPU for 4k Transcoding

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What @ZeroCool said. I'd look to plan for what can transcode your media most effectively/efficiently. At this point, the best bang for your buck you can get is what I'd go for personally. I like to avoid transcoding if possible, even though my server is more than capable. Get something that can take your media to whatever format works with your clients and you have to worry much less about transcoding.