Well, 4.0 was free for a limited time only too - DAZ is just making sure it can end the offer when it wants, not setting, as far as I know, a specific limit. And it will go on working once you have it - the end of the free period won't disable your serial number. As for whether it's worth getting, 4.5 fixes a lot of bugs and adds some useful features (instancing, for example, which is useful for populating a scene if you want multiple copies of an item, and AutoFit now preserves morphs), as well as having a finalised and documented version of the DSON format which will work with Carrara 8.5 and should, in theory, open the door to other tools working with native content.