You can't get the older version of Lightroom, unfortunately. Just a little note: if you have a newer camera, the older Lr 4 may not support it.
Which operating system are you currently running?
Windows Vista. It's a desktop that hasn't been used much for years, but now I need it for editing on the two large monitors (our other conputer is a laptop). I plan to upgrade to Windows 8.1, I'm just not sure when.