Hi Kevin,
Notation Composer 4 does not directly support VST instruments, but if you have a host for your instruments/library (eg. Garritan's Aria player) and a virtual MIDI cable(s) (eg. LoopBe) then you're set.
The player acts as the MIDI Playback Device - please
see this thread in this forum for details. There are also
some good tips in this thread too regarding other VST hosts.
Hope this helps!
Sherry