Finally, I was able to find a workaround for the problem. Anyone can use TinyMCE with JSF 2 and RichFaces 4. You just need to add few lines:
save_onsavecallback : "myFunction", [Use this comma properly.]
This line should be added to the init method of TinyMCE. Then, create a function named "myFunction" or any name you want. This function will allow TinyMCE to update the textarea with latest data. Finally, call this function from onClick of the commandButton. <a4j:commandButton onClick="myFunction()" />
I use the editor the one comes with JBoss providing visual/source/preview.