red and blue sub pixels on the Note 4 are only 366 PPI, while white and green subpixels are 511 PPI.
Its the effect of Pentile
Yeah, nothing confusing about it. "Full RGB" means that each pixel gets its own R,G,B subpixels instead of sharing. With any shared layout (diamonds or not) you have fewer red and blue subpixels, so you're only really getting the advertised resolution on green. Considering the replies above their marketing seems to be working (holy crap, diamonds).
The easiest way to see this effect is to compare aliasing on a Note 3 and an iP6+ (or another full stripe 1080P panel of similar size). They advertise the same resolution, but the full stripe 1080P panel is much sharper in practice unless you only compare green.
Basically an RGB 1080P 5.7" screen is, what, 388 ppi? The effective ppi of the Note 4 screen is (366+366+511)/3 = 414. In practice this number is just an estimate since you get the full 511 if you have all green and the worst case (366) if you have all red/blue.
In terms of reading text this works out pretty well. In that case you're talking about black on white and the green pixels lighting up to 100% around the black text more or less means the text does appear at 511 ppi.
In terms of rendering for games or displaying images it's more of a mixed bag and in that case the advantage over 1080P RGB is limited.
So I stand by my original statement: Samsung would get much better results going to full RGB 2560x1440 and saving some GPU power vs doing pentile 4K. Their marketing department wants to see new numbers so it unfortunately won't happen.
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Just want to reiterate that the Note 4 display looks amazing, no questions. It's just that rendering at 2560x1440 for pentile is wasteful on the GPU end (rendering doesn't care about the subpixel layout).