You can simply delete the catalog file and the associated preview folder.
To find its location select Help / System Info while having the catalog open. You will see a line similar to this:
Library Path: C:\Users\xxxxx\Pictures\Lightroom\Lightroom 4 Catalog.lrcat
and this is the name of the the current catalog as well as its location.
More information on file locations:
You don't need to export the catalog from LR3. Simply open it from LR4 and LR4 will convert it to a copy to work with.
Message was edited by: F. McLion
I have done that, and it works. Thanks, having made one mistake I didn't want to make it any worse).