If Apple releases a Retina version of the old high-res 15", that would be 3360x2100, the resolution they should have standardized by now on the 15" MacBook Pro (hopefully it'll happen next year). Still not enough to display 4K video 1:1 though, especially when taking the horizontal resolution into consideration.Though, since you opened that bottle, I'm not sure if I'd call it the same situation, for a number of reasons. For one, if Apple were to use the same pixel density that they have on the current 2018 MBs/MBPs for a 17" one (namely, 220dpi), then we would likely still end up below 4K, presumably at around 2000 pixels vertically (by the way, wouldn't 4K be 2160p, not 3840p like you say, as this number derives from the vertical, not horizontal amount of pixels?). So it would be closer to 4K than we are now, but still not quite there. One could make an argument that Apple could just depart from their current pixel density on a hypothetical 17" model either way, but then again, if that were to happen then we'd also get a resolution bump in the 13" and 15" models aswell, likely pushing (at least) the 15" model up to or above the 4K barrier and making that argument irrelevant either way.
The old 17" MacBook Pro was 1920x1200 at a comparable pixel density to the high-res 15". The Retina version would be 3840x2400, which is enough to display 4K video 1:1.
You're right, I meant 4K = 2160p (vertical resolution), or 1080p x 2. My mistake.
The Retina display is not yet to the point that a bump in resolution isn't noticeable, at least for those with better eyesight. The difference is less for compressed video, but no doubt UI elements look less crisp to me after setting my MacBook Pro to the 3360x2100 software resolution, while the panel's physical resolution remains 2880x1800.Secondly, most people would find the jump from 1800p to 2160p not nearly as large as the jump from 900p to 1080p – our current "Retina" screens are already above the threshold where the average human eye can distinguish between individual pixels at a normal screen distance, which is not true for the jump from 900p to 1080p.
The same was true with 1080p back in 2008, the year Apple made 1920x1200 standard on the 17" MacBook Pro. In fact, most of the content on sites like YouTube was still 480p or below.And thirdly, I'd hardly call 4K the (one and only) standard for video content – it's one standard, alongside 1080p, 1440p and so on. There is still plenty of video content that is only available in sub-4K resolutions, for a variety of reasons. Many streaming services like Netflix have 1080p as a standard option for subscribers (with a premium-tier with 4K content above that, and even then, a large part of their library just isn't available in that resolution), a large portion of videos uploaded to YouTube and other video sites are still "only" 1080p or 1440p. And so on and on. For a great deal of people, the 4K content that they watch makes only a small fraction of the video content they consume in total.