There are many games that will be out for both PS3 and PS4 (or Xbox 360 and Xbox One) that may not look that much of a difference. Ignoring first of all the idea that with each generation, graphics will look less noticeably different because of what the human eye can perceive, it is also a matter of cost.
This of the difference between a PS3 and PS4 as being a low spec PC and a high spec PC. The low spec can still handle the game but with worse graphics, less pre-rendered textures etc. While the high end can also handle the same game just with a high finish.
For most PC games though, it's made and rendered to the high end PC, not low end. For AC4 I think it was made with PS3 and 360 in mind and they 'bumped it' as such to PS4 and One levels to get the extra sales from it. It's a matter of the amount of effort and money they are willing to put in for the return they get, in this case, 140 million market vs 2 million max. They might as well spent lot of time making sure the PS3 and 360 ones look good and just make sure the PS4/One look better than build it for the PS4/One, costing more money and then lower it for the PS3/360.
Now, I've said all, this but in reality, I didn't get your opening point...
Hmm, pie.
