Original sounds + some user contributions downloaded from Native Instruments.
Either replace ~/Documents/Native Instruments/Shared Content/Sounds with this repository:
# git clone git://github.com/gudleik/Guitar-Rig.git ~/Documents/Native\ Instruments/Shared\ Content/Sounds
Then Rescan presets in Guitar Rig 4.
Or check out the repository somewhere else and cherry-pick the sounds you want by dragging them into Guitar Rig.