There is a bug with the recent OS X Mountain Lion Update v10.8.4 whereby if you are a Standard User (non-Administrator) you will not see 10.8.4 listed as an option in Updates within the App Store. The fix is to either login with an Administrator account or...
- Open App Store and click on the Updates tab.
- In the Store menu select Reload Page.
- Enter your admin credentials.
- Updates should now appear.