Your CS5 would not have the Ext goodies, but if you left CS4 Ext on your computer you could use them in that package. You would have to save out your CS5 work in a CS4 compatible file.
If you want the functionality while working in CS5 you will need to upgrade to CS5 Ext.
Thanks, that is what I thought but an Adobe rep that I spoke to last night said I can use the Ext functions from CS4 Ext with the CS5 only upgrade. I have not been happy with Adobe about CS5 release, I bought CS4 Ext on March 3 and then found out about the CS5 release shortly afterwards and they would not allow me to get the free upgrade. There is no way that I would have bought CS4 had I known that CS5 was right around the corner. Great products but very stingy marketing! Other successful corporations have a lot more respect and courtesy and go above and beyond the bare minimum to keep their customers happy.