Lightroom 4

Does anyone know if it is possible to get permits for Lightroom 4 anywhere? i have tried through Adobe but they only appear to be doing the new version. This would be fine but Lightroom 5 requires Windows 7 as a minimum and unfortunately my laptop is running XP. Any help/info would be great.

I have already downloaded the software but can seem to find anywhere to buy the licence key online.

Cheers,
Ben