Don't look now, but Sibelius 6 has been released.
It has a host of subtle improvements that should make editing and engraving much easier. They have also added new features for guitarists, which I really appreciate.
More importantly to this forum, it doesn't use Native Instruments Kontakt Player for virtual instrument playback anymore. It uses a player from ProTools, and provides a General MIDI sound set from M-Audio (Avid Technologies now owns Digidesign Pro Tools, M-Audio and Sibelius). It still has sounds from the Garritan libraries. And it still supports VST and AU (Apple Audio Unit) instruments, so I imagine that GPO4 with Aria will be compatible.
I have placed an order for an upgrade but I expect it will be at least three weeks before it ships.
There is a demo (without the sound libraries) that you can download and try out. It does everything but save and export.