Re: Should I upgrade from Finale 2014's Garritan Sounds to GPO4?
The short answer is yes.
You will need to need to set the soundmap priority to GPO4 in the MIDI/Audio menu or manually monkey with the instrument assignments in the Score Manager, which can be a little tricky to tot things exactly as you want them.
It's quite a broad selection in the set for Finale, with many instruments gathered from the other Garritan Libraries - JABB, World, COMB and Instant Orchestra. So it has quite a few instruments that aren't in the full GPO - such as the saxes, acoustic guitar, and ethnic percussion. But you can see from the list that GPO has the traditional orchestra covered more completely.
If you're asking if the GPO instruments are an improvement in the quality of those in the Finale list, that wouldn't be fair to say since the GPO instruments included in Finale are the same.