The C4 Engine is a comprehensive suite of professional, robustly implemented game programming tools for PlayStation 4, Windows, Mac, and Linux.
I am updating my review. (Oct 2014)
As an engine, and programming interface, C4 is superb. It's been built and continually refined by Dr Eric Lengyel, a highly trained mathematician and computer scientist. C4's engineering is probably, and I do mean probably, the most academically sound game engine ever constructed. Sometimes I feel like I'm reading Shakespeare reading C4 code.
If your programming toolbelt is highly diverse, it will take adjusting to C4's environment. It's highly self-contained. The C++ STL isn't involved anywhere, and essentially all its components were implemented from scratch by Eric. There are clearly advantages and disadvantages with that.
As an artistic tool for translating the pictures in your head to what's onscreen easily, it's a bit of a friction generator. Every release improves this aspect some, and Terathon has expressed an increased focus to its artistic toolset / chain for the next few versions.
Eric is a very good coder and has made a very nice engine, the code is clean and professional and the engine is as stable as rock. But unfortunately the tools aren't very artist friendly, they don't support multi screen setups, many developers use more than one screen so this is a major pita for fast work, instead of working almost exclusively on a 3D view it requires you to perform some tasks on the 2D orthographic views more times then what is desirable, the window where you pick and search for your models and game objects (called nodes in C4) is a very complicated and strange hierarchy tree like system, it should be much simpler then that, i will not mention other engines but theres much better implementations of this on other engines.
Eric is also a coder that tries to put too much stuff in the same basket, what do i mean by this, for example he tries to make all himself without using any other 3rd party software on his engine, for example the physics engine, Eric instead of implementing a already made physics engine, like the majority of his concurrents, he made his one, it makes for a better integration and support is true, but unfortunately it slows down the development of other important features, and the physics aren't that good, objects don't behave realistically, for example theres no self rotation when an object is flying from a explosion, and sometimes the character controller becomes stuck at the boundaries of floor geometries. Because of this Eric is slow at implementing more visible stuff, things that could make the engine stand out, HDR that is being promised for a looong time now but is being pushed further and further away, the grass system and the underwater capabilities that could make the engine really stand out are also still to be made, etc, Eric prioritizes stuff that don't really give much exposure to his engine and because of that, a very good engine is being brushed aside for engines imo much worse.