My experience has been that Adobe only sells current versions. As far as I know, you will have to buy from another vendor if you can find one that still has Lightroom 4 available.
Thank you, Jim, for your prompt reply. Amazon has it for $151.99, Ebay has a few copies that end up being about as expensive after international shipping and other fees, and I don't trust most of the other sites after reading reviews on them. Looks like I'm going to have to fork out some dollars. How unfortunate.
The Amazon price isn't really that far out of line for a first-time purchase. I realize that the Adobe price was $149.99. If you are looking at the $79.99 price tag from Adobe, that is for an upgrade version which requires that you have a serial number from any previous version in order to validate the installation.