It's very hard to decipher what you mean (if English isn't your native language, I understand how there can be confusing phrases), but I think what the problem is that:
1. You have Mountain Lion, but some earlier version than 10.8.4 (like 10.8.0, 10.8.1, etc)
2. You wanted to update it to the latest version, 10.8.4 (this would be a free update)
3. You (understandably) went to the Mac App Store and tried to download it from there, because of course the one that is there is now 10.8.4. Perfectly understandable but incorrect.
4. To update an existing version of OS X, you use the "updates" section of the store (see picture below), not the actual "OS X Mountain Lion" page. There is one exception to this: if you are trying to tell us that your machine came with a pirated version or someone else's version of Mountain Lion, then yes you will need to buy your own copy of Mountain Lion and install it, and THEN you will get the free updates such as 10.8.4 and the future 10.8.5 and so on.
If you already knew all this and I misinterpreted your question, my apologies.