The PS4 is not going to be a True 4k console. And i think that is exactly what sony intended.
Make a console that has the power to do stable 1080p and extra power to dial up everything within that 1080p then output that in 4k with an upscaling alogarithm that has more power then the average upscaler has.
What you end up with is something that looks better than 1080p but bot as good as 4k, but a ge that has more IQ improvements outside just resolution.
Sony will be relying on AMD's upscaler.
What you describe is a scenario that could also very well occur on Scorpio'. It's really all up to the developers, so it's not really a selling point for Sony, nor does it make the Neo magically better than the competition.
Besides, the vanilla 'launch' Playstation 4 and Xbox One can already upscale to 4k, the hardware scaler has it built in today and has done so since these platforms launched, the only caveat is that it's limited to 30fps/hz as per the HDMI 1.4 standard.
At the end of the day though Scorpio is likely to have better looking multiplats.
Oh yes.... i expect thats what MS is going to do too. Render games internally at 1440p then upscale that to 4k. it the dame thing they hwbe done with the XB1, render games internally at a lower rez then upscale. Their upscaler just had a loy of halo artifacts and thats what resulted in the "high contrasty" looking XB2 games.
And not likely, i expect that the scorpio is gauranteed to habe the better looking multiplats. though when it comes to rez the diminishing returns thing is more obvious. Big differemce going from SD to 720p. Not as much from 720p to 1080p. Even less from 1080p to 4k.......etc.