4.22 (also known as 4.2 maintenance 2) is the version that is currently supported with hotfixes as needed. It was released in Sept 2009, and since then has received periodic hotfixes for customer reported issues. If you run into a problem that requires a fix, you would have to update to 4.22. Here are the documented fixes *since* 4.22:
http://ftp.sas.com/techsup/download/hotfix/HF2/A53.html
There were also a number of fixes within 4.22 that aren't listed here.
If you are installing on newer operating systems (Windows 7, Windows 2008 R2), then you will want the most recent versions because the newer installers are more "aware" of those systems and how to ensure system requirements are met.
4.3 is due out this summer. Not quite as soon as "within the month" as Doc indicated, but I still feel good about saying "summer"...
Chris