In a big region of 16 cities there are 4 great works to build but not every city can built them, just the four which surround them. Will the Great work bring benefits to the entire region or only to the cities surrounding them?
It is only to the 4 cities that built it. Sorry :(
The 16 city regions are broke up into clusters of 4 cities each. Each 4-city cluster does NOT even "see" or notice the other cluster's Great Work. Let me show you an example:
I have a Space Center Great Work in the red highlighted portion, but my current city does not even recognize it as being built, because its in a different cluster of the region.
There are regions with more than 4 city cluster to 1 Great Work site, however. Whitewater Valley (5 cities to 1 Great Work) and Reflection Atoll (7 cities to 1 Great Work) both exceed that number.
This is all very contrary to the wording on some of the descriptions of the Great Works (saying that they benefit the entire region).
Also, I do believe most of the Great Works' benefits arrive by road, and that's the main separation reason of the clusters, highway region roads.