4x PPI would be 16x pixel density.  So 4x PPI means a UHD display the same physical size as a Full HD display but 16x the pixel density.  And that should make for a much *sharper* image, not at all stretched out or pixelated – quite the opposite.

Just as an SD image on a small HD display doesn’t look stretched out or pixelated – if the image processing is good anyway.  It is only when you *physically* enlarge the display that it starts becoming an issue.  Because then the granularity of the image source becomes an issue.

A 61″ UHD display showing Full HD content would probably look as good, if not better, than my 61″ Full HD DLP, or any 61″ Full HD display.   Again, provided image processing is good to properly populate those dense pixels.  But if you start making larger displays, the pixels become more evident.  That’s why you need to sit farther away from large displays.  I can sit a couple of feet from a 1080p laptop display and HD video looks good.  But if I sit the same distance from my HDTV I can see pixels.  Ten feet away it looks fine.

I’ve seen 100+” HDTVs at CES, and at 10 feet I can see pixels.  With a 100″ UHD at ten feet you probably wouldn’t see the pixels, but the image detail (given the same HD source) would be the same.

Same effect with the retina display being that much sharper than the old display at roughly the same physical size.  They just upped the pixel density.  The same content looks better on the higher density display.

As for the compression, I did point out that it would be worst case scenario and in the real world it isn’t a linear scaling.  Since compression operates on ‘blocks’ of an image if you take the same image and just scale up the resolution you’ll have larger areas of the higher resolution image that are the same, or similar, and can be better represented in a compressed format.  Just about the worst case for compression is random noise – like an analog tuner receiving nothing, showing random dots.

Also we know (and you know) the data rate doesn’t scale linearly with the number of pixels.  If we look at Cable/MPEG-2 VOD streams, a 1920×1080 image has 6 times the number of pixels vs a 720×480 image but typically is encoded at 15Mbps vs. 3.75Mbps so 4X.  Or for an h.264 comparison maybe AT&T’s 7.5Mbps HD vs. 2.5Mbps for SD ratio of 3X…

Personally I’m more interested in something I might see sooner rather than later, meaning 4K displays.  I’m assuming we’ll actually see some 4K activity at CES this coming January, especially wrt those passive 3D panels.  Would at the very least allow them to deliver “Full HD” resolution despite the loss of half their pixels…