Lightroom 4 -
There are a couple of options for downloading Lightroom 4:
Mac: http://www.adobe.com/support/downloads/detail.jsp?ftpID=5566
PC: http://www.adobe.com/support/downloads/detail.jsp?ftpID=5567
Lightroom - all versions
Windows
http://www.adobe.com/support/downloads/product.jsp?product=113&platform=Windows
Mac
http://www.adobe.com/support/downloads/product.jsp?product=113&platform=Macintosh
The page linked below should also have links for Lightroom 4:
http://prodesigntools.com/adobe-lightroom-4-is-out.html#ddl
Be sure to follow instructions regarding signing in or else the downloads will not process properly. If you visit one of the other download pages in that site they have a section called "Note: Very Important Instructions" that gives specific steps for signing in.