Due to having PowerPC machines we're unable to use Photoshop CS5 so we want to upgrade as far as we can, which is CS4. The Adobe store doesn't seem to sell CS4 any more, so we're looking at buying through a third-party vendor. Vendors sell both CS4 Full Versions and Upgrades; would we be able to use an Upgrade version with a CS2 serial number? Thanks!
An upgrade should work for you, because you are less than three versions behind CS4, which is Adobe's usual limitation rule.
Just be sure you get it from an authorized reseller - there is a lot of fake and damaging software out there...
See "Find a Reseller" near the bottom