Is it possible?

Sure, if you've got the source there's nothing stopping you.

C4 does not, in itself, contain MMO networking or entity support -- the support it has is for smaller games. However, there is nothing stopping you from adding an MMO data and networking model (as well as back-end), and using C4 for the front-end and rendering.

ok thanks guys just wanted to know if it was possible

is it very difficult to do?

The hardest part is building the tools and infrastructure to support the development, production, deployment and operation of an MMO. However, you can buy/license much of this technology.

The second hardest part is coming up with a game design that hasn't been done to death already, and where you'll actually find a market big enough to support the substantial investment needed.

The third hardest part is actually producing the game, as there's a ton of content needed. Large teams of artists all need leadership, style bibles, salaries, healthcare, office space, computers, etc for a long time.

The fourth hardest part is to then operate the game, keeping it up at all times of the day, fixing exploits as soon as they're found, retaining engineers for doing maintainance and expansions, etc.

If you will make a MORPG (note the lack of "massive") then it becomes possible for an indie team to do it, if the team has previous experience, understanding spouses, and two years of spare time on their hands.

If you know your tools and understand the underlaying structures then you have a good chance to do it, but beware of the factors "TIME" and "MOTIVAION".
To beat "MOTIVATION" (keep motivated) is a personal thing and depends on your talent and discipline.
There are some solutions to beat "TIME", e.g. use lots of stable and mature middlewares :
just an example:
- Networking (RakNET, ...) -> Server + Client
- Scripting (LUA, GameMonkey, ...) -> Server + Client
- Sound (FMOD, ...) -> Client
- Secutrity (CryptoPP) -> Server + Client
- Database (SQLite, SQL Server+ADO, ...) -> Server

and of course on the client-side the type of game you want to present, in case of 3D you got tons of options (Ogre3D, C4, ....).

As a first step i would suggest that you just put all the middlewares (just like a puzzle) together and build a "framework" and make some prototypes in this framework and "adjust" it until it meets your needs.
Then the "fun" can begin, you start to create and put content. If you choose your middlewares wisley you "almost" don't need to program many "tools" to put content into your framework (making tools is very very time consuming).

Build and testing such a framework took me ~2 weeks to accomplish!
But now comes the most difficult part of a game : the GAME itself and this has nothing to do with 3D engines, special effects, etc.
Thats the mistake most ppl do on web, they mean GAME and always talk about GRAPHICS (3D) and fail with their mission.