GPO4 has more sounds (and more options within instrument groups) than the included Finale Garritan sounds (GIfF sounds).

https://makemusic.zendesk.com/hc/en-us/articles/217663758-Setting-up-Garritan-libraries-with-Finale-and-PrintMusic 

Although this link deals with setting up GPO5, process should be similar to setting up GPO4.

If you have trouble finding the setup files for GPO4, use the Submit a request link at the top of this page to see if MM will provide them for you.

 (Printmusic never came with Garritan sounds, as far as I know, but I could be wrong.  It did come SoftSynth sounds.)

Correct on both counts. Since you have GPO, GIfF is redundant but always install it so that 25 can find the Aria Player.

GPO 4 owners also have a $49.95 upgrade path to GPO 5.

https://www.garritan.com/products/personal-orchestra-5/

Thanks for the information!  This puts everything into perspective; I'm sure I could find a use for the oddball GIfF sounds (whatever that stands for), and thanks for the tip about the Aria Player.  I like the new one, and did not quite join the dots as to where it came from, obvious in retrospect.  I did successfully get my old GPO4 sounds all hooked in, and finally, thanks for the tip about upgrading to GPO5!  I'm retired now, and need a little artificial complexity in my life.

Ptolemy, formerly known as Arch

Garritan Instruments for Finale (GIfF/GIFF) can only be used in Finale. They take up, what, 7.8mB or so. If you don't have GPO or other sound libraries, they're very handy.

Also, even if you have GPO, GIfF sounds include a selection of sounds from other Garritan libraries (JABB, GWI, etc.) which you may find handy.

Thanks for both the previous responses; I did notice the unexpected sounds popping up, and wasn't sure from whence they came!