The Mac draws on a 5K canvas and then downscales that for the 4K panel, so it is taking advantage of the full 4K resolution of the display. There's the same amount of workspace as on a 27" 2560x1440 monitor, but with more detail than an actual 27" 2560x1440 monitor would offer, and less than a 27" 5120x2880 monitor could display. More @Wikipedia
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