Steve,
WebAssist's policy has typically been that current versions of their extensions will be supported on the most recent Dreamweaver and ONE version back. In the case of UE4, because it came out late in the cycle of Dreamweaver CS4 and is now being updated for CS5, this is particular case where the update will probably support CS5, CS4, and CS3.
I am not sure I would try it with Dreamweaver 8 unless a WebAssist engineer or employee could verify that it could work. I'm not even sure that DW8 has the necessary extension engine to power the new features that were packed in UE4 - so it's not that WebAssist doesn't have backward compatibility in mind but more so a limitation of older software and WebAssist's desire to be on a more current approach.
Regards