Ardour and G5 quad

Does Ardour use all of the quads processors?

Ardour is extensively multi-threaded so it will use the processors for a combination of GUI, audio processing, and disk I/O. However, the realtime audio processing thread will only ever use one processor, so you won’t see a benefit in plugin count with multiple CPUs. That said, the interface will be much more responsive on multi-processor/core systems with large plugin-heavy sessions.