Four years is relatively a 'long' period when it comes to hardware/software. Adobe's CS5 suite requires 64 bit CPU as well as 64 bit OS ( Vista/Windows 7) with reasonable amount of RAM specifically to capitalize on the software's capability to make use of the additional Cores in the Processor and also to fully make use of the available physical RAM.
When I bought the CS5 Production Premium suite from Adobe, I didn't factor in the additional cost to upgrade the PC. Finally I ended up upgrading the PC and had to order a Windows 7 Ultimate - just to run the Adobe suite. CS4 doesn't have this minimum requirement of a 64 bit hardware/OS.