By the way, I would like to know why, after I trashed the preference file and restarted LR4, LR 4 created a new LR4 catalog in a different location from my existing LR 4 catalog, as well as incorporated my LR 3 catalog (I still have LR 3 on my computer as well) into this new LR 4 catalog.
The most recently-used catalogue file is recorded in the LR4 or Lr3 preferences file, so destroying it means that when you restart LR you will be presented with a dialog box as if you have just begun LR4 and it has discovered the Lr3 preferences. So it'll ask you to upgrade the most recent Lr3 catalogue or create a new catalogue in the default location, which is what you must have chosen. There is also a Choose button which you must have overlooked. It would have been easier to have double clicked your LR4 catalogue to restart LR.
Incidentally, in my view people are too quick to recommend destroying the preference file. Corrupt preference files are more of a Mac problem, and I wouldn't expect them to be a problem in Import. It's usually user error