I believe I am right in saying that the only additions to the content in Groove Agent 5 are The Kit (Acoustic Agent sounds and styles) and Raw Power (Beat Agent sounds and patterns). The Kit is huge and accounts for most of the growth in the size of the content bundle. This is certainly how I understand Steinberg’s “New in 5” page.
There might be improvements or bug fixes in some of the Groove Agent 4 content bundle, but I haven’t made a detailed comparison of the files supplied with both versions of Groove Agent.