It's odd - on the Sibelius chat page, they list a lot of the update features, but not GPO 4 (there is also a pdf you can download with the complete list of changes); they do, however, mention it on the blog page (link above). Anyway, I downloaded it, and it works as one would expect. And, as usual, they update - .1, .2, .3, etc., is free.
Now from one Sibelius user to another: Should I download the upgrade? does it function as well as the old KP2 version? Keep in mind I tend to do things like edit my percussion instruments so I can use the bass drum roll sound, and change my tremolos to +tremelo.unmeasured. Does all that stuff work?
I don't think there is a problem in having both - GPO 4 only works with ARIA, and whatever GPO you did in Sibelius with KP 2 will still; the only catch is that you have to have both GPO libraries on your computer, and that takes up some acreage...
I haven't given ARIA a big test-run (busy...), but it seems to work as advertised. And, some of the other features in the update make it worthwhile.