fascinating puzzle - but where is the 4th dimension?

The puzzle consists of three parts, the first and lowest layer is a 'normal' puzzle with the historic Paris on it. It was relatively hard to put down, as many parts were very similar from cutting ago and barely distinguishable from the colors here were. That took some time but after 2 days it was finished.

The next layer is then plastically, a foam is imprinted on the modern Paris. That went pretty fast and you put it on the bottom layer.
From the foam one takes then punched out in the numbered parts to insert the most striking buildings in the city as a third Dimenion.

All in all, for puzzle lovers a worthwhile investment which is not too difficult, the result is really fascinating.

Where, however, is the fourth dimension is not obvious to me, but 3D is indeed ok!