Some would say that with some decent plug-ins you can master your projects in the box just fine. Others would argue you need to send it to someone with various studio grade analog hardware processors.
The truth is probably somewhere in between. And it really depends on your expectations and definition of "mastering." And it certainly is possible to get some pretty amazing results.
But that said, MC4 is pretty old. MC5 was replaced quite a while ago by V6. At the very least, open your program and click on the "help" button in the toolbar, and select "check for updates."
I am not sure how far that will get you. If its the final build of V5, thats a big leap in content and function.
Be aware also that an upgrade to the current version of V6 is very affordable and yet another big leap.
(Greg beat me)