FileNet introduced an entirely new API architecture with their P8 4.0 release. Many people refer to this new API often as FileNet-4-API while the old architecture is called FileNet-3-API. The problem is that the old API architecture is still available on FileNet 4 and even on FileNet 5, although not supported.
After digging in the documentation, I found the internal codenames used by Filenet for these APIs:
P8 3: Buzz API
P8 4: Daphne API
Using these codenames to refer to an API architecture is more clear than using this "FileNet-4-API" term.
FileNet 4 is delivered including the Buzz API contained in javaapi.jar. In the case of a FileNet 4 Content Engine, it does not establish a WebService connection by itself but uses the nmew Daphne API to simulate the RPCs. Interestingly, you can find an early version of this compatibility mode already in the Buzz API delivered with FileNet 3.5. So FileNet must have been working on this new Daphne architecture for quite a while now.