The C4 Engine is a comprehensive suite of professional, robustly implemented game programming tools for PlayStation 4, Windows, Mac, and Linux.
I went through two other engines before I settled for C4, Irrlicht and Ogre. I wasn't necessarily looking for a free engine but at the time (7 resp. 4 years ago) there wasn't much choice.
C4 simply works and if something doesnt work, it's a good chance it's you doing something wrong and not the engine. In that case you will almost always get help in the user forums, the community is very helpful and the developer also answer questions regularly there, it's one of the features that really sets this engine above the others.
That said, I reversed my original rating from 10 to 4. I save myself the bother of repeating everything that Argoon has said in his review, because he sums up my own criticism after over a year of C4 usage very well.
Writing an application outside the given framework is possible but with a lot of woe. You wont be able to write stand alone tools for instance, only tools that use the engine GUI and start from within the engine main frame.This means in the end that you have to keep and maintain several copies of the data directory hierarchy.Also dont expect any help when you do. Help is provided in the forums, but you have to use the engine the way it was intended by the developer or you're on your own.
There is not much documentation on the library,in order to do anything beyond the obvious you have to go to the forum and ask for help and hope the developer is in the mood to help. He may or may not. The engine contains absolutely no source code comments and the included header comments often just repeat the function name and parameters and otherwise leave you in the dark. In fact it is impossible to write any non trivial application without asking for help in the forum over and over again. As for the community: If you criticize the engine in any form,you will be yelled at and silenced pretty quickly.
Facit:A stable engine,alas usability leaves a lot to be desired.