Rodger, Dan - hi there guys.
As far as I can see, there appears to be no way to automate the 'master' bus in GT4, or to bounce down to a single track, but there are two workarounds to your problem which are both quicker and less fiddly than fading each track individually.
The first method will work well if your project only has audio tracks, the second works with a project with both audio and MIDI, it also allows much more control over how your song fades out.
METHOD 1 - Fade out in the same project For this method to work properly, all your tracks should end at the same point. When you record, don't forget to leave each track running until
after the point where you want the final fade out to finish - set a marker so you know to carry on playing until you've passed it.
I've set up this project to show what I mean - this screenshot shows the end of a song that contains five audio tracks. As you can see, I've already placed a marker at the start of measure 62 to give me a rough idea of when the fade will start, and I've also inserted another marker at the beginning of measure 70 where I want the fade to finish.
As you can see, every track is recorded past the end of the song; this is important as you want
all tracks to finish at zero level/volume
at the same time (otherwise some of your tracks will still be audible while others have already been silenced).
It doesn't matter if all the tracks don't finish at the same time - make sure
Snap To Grid is turned on and set to 'Whole' or 'Measure'.
Now place the Now Time Marker at the point you wish the song to end - in my example, this will be at the second marker (at the start of measure 70).
Now you need to split each track at this point - to do it quickly, use the keyboard shortcut
Ctrl+A to select every track. To show they've been selected, all the tracks will change colour:
Now you need to use the keyboard shortcut
S (just the letter 'S') - this will split all the tracks together.
You now want to delete off all the parts of each track that you've split off at the end. You can right click in each of these and use
Edit>Delete or the
Delete button on you keyboard. However, a much quicker way is to hold the left mouse button down and drag out an envelope to enclose all the clips:
Now press
Delete on your keyboard and all the extra clips will disappear:
Now use
Ctr+A to select all your tracks again.
Hover the mouse cursor over the top right hand corner of
any of the tracks until it turns into the Fade Icon.
It looks a little like this
Holding down the left mouse button, simply drag away to the left and you'll create an identical fade envelope for all the tracks at the same time:
METHOD 2 - Fade out in a different project Method 2 requires you to export your mixed song, and works if you have MIDI in the project – note that using this method, you don’t need to trim all your tracks to the same finishing time.
To do this, use
Ctr+A to select everything in your project and then go to
File > Export Audio - on the screen that appears, navigate to a destination folder, name your project, and make certain you have
Entire Mix - CD Quality selected as the Export Preset:
Now open a new project in GT4 and click on an Audio Track (Track 1 is fine) to select it. Now select
File > Import > Audio and navigate to your mixed-down song. Click on the song to select it and then hit
Open to import it into the new project.
To apply a simple linear fade to your whole song, simply use the same method as above and drag the Fade Icon in the top right corner.
You can, however, use this single track method to create more sophisticated fade-outs.
To do this, first you need to create what’s called a
Volume Envelope – to do this, right click on the track and select
Envelope > Create Track Envelope > Volume 
The Volume Envelope will appear as a coloured horizontal line in the centre of your track:
By default, the level of the envelope is set to minus 7.1 decibels (-7.1dB) – you’ll need to left click on the envelope and drag it up until the level reaches 0dB (this means the original volume of your song is unchanged):
Now you need to right click in the track at the point you want the fade to start – from the options available, select
Add Node 
The node appears as a ‘diamond’ icon on the envelope:
To create the fade, left click on the (automatically created) node at the very end of the envelope (at the right hand edge of the track) and drag it down until it displays
–INF (minus infinity). As you can see from this screenshot, a linear fade out is created (note you can slide either node horizontally to change the start and end points of the fade):
To quickly create a couple of different fade out characteristics, right click on the fade envelope and choose either
Fast Curve or
Slow Curve - here’s the Fast Curve:
To exactly control the fade out, you can add more nodes anywhere along the envelope and drag them up to increase or lower the relative volume:
Finally (nearly there guys!) – using this single track import method, you can use your VS-20 to manually create a fade out envelope in real time.
To do this, go to the Console View and arm the track to record (make sure you’ve got nothing connected to the track’s input). Now click on the
W button to enable Automation Write:
Assuming the imported song is on Track 1, first make certain the fader on Track 1 on your VS-20 is set at 0dB as your starting position. Now switch back to the Track View and hit Record. You can now use the fader on your VS-20 to create a volume envelope in real time as the track plays.
Note that the nodes that the Automation Write process inserts in your track are fully editable as in the method above.
Incidentally, both of these methods work equally well for fade-ins.
Anyway, hope that helps guys – it actually looks a lot more complicated than it really is. Work through each method to see if they work for you – let me know if there’s anything else that needs explaining.
Steve