I made a test with occlusion culling in Unity. Here you can see a scene with some houses and my camera in front of the first house (looking at the wall, hiding the other ones).
Without occlusion culling (only view frustrum culling) it looks like this:



Here is the same scene with occlusion culling enabled:



This really saves some render calls. Something similar is possible in C4 also. But there you have to place the occlusion geometry manually. Unity uses pre-calculated PVS data to lookup where culling occurs. So it is absolutely easy to setup.