EDIT: Updated, see Below.
I have been having a lot of issues with the digital ins and xruns, I will try this next week when I return home from visiting my hometown and see how it goes.
Here a post:
I have successfully enabled 4 ins with what Paul explains there, but that persistent xrun I have no been able to track it down from where it comes. If I only use the 2 analog ins everything works great.
Thanks for this tip.
not sure if somebody posted a similar howto somewhere… at least i didnt find anything…
i was looking to have 4 outputs at the same time which was kind of tricky since the fast track pro natively delivers only two stereo outputs (A and B) to choose from.
with a little alsa hack it worked for me 
put this to .asoundrc in your local folder
be aware that your hardware address might be different (according to the ouptput of “cat /proc/asound/cards”)
the lines marked with ### eventually need to be edited!
pcm.mt4 {
type multi
slaves.a.pcm "hw:1,0"; ###
slaves.a.channels 2;
slaves.b.pcm "hw:1,1"; ###
slaves.b.channels 2;
bindings.0.slave a;
bindings.0.channel 0;
bindings.1.slave a;
bindings.1.channel 1;
bindings.2.slave b;
bindings.2.channel 0;
bindings.3.slave b;
bindings.3.channel 1;
}
ctl.mt4 {
type hw;
card 1; ###
}
pcm.multi4 {
type route;
slave.pcm "mt4";
ttable.0.0 1;
ttable.1.1 1;
ttable.2.2 1;
ttable.3.3 1;
}
ctl.multi4 {
type hw;
card 1; ###
}
now start jack with:
jackd -R -d alsa -d multi4 -r 44100 -n 3 -p 1024
and voila… you should now have 4 in and 4 out to choose in qjackctl or ardour 
and they are even synced (as far as i could find out both stereo outputs use the same “clock”)
hope it will help somebody 
btw: my distro is archlinux with kernel 3.2.11
I found another way to do this today. I got it from the first page of this writeup: http://www.noid.net/hydraplayer/docs/08_README.cloning
Here is a summary:
-
In qjackctl, open the Patchbay
-
Create an Input Socket for your default output (playback_1 and playback_2, in my case)
-
Create another Input Socket for the second set of outputs you want to use (playback_3 and playback_4, in my case)
3b) Create a Forward from Input Socket 1 to Input Socket 2 -
Save the Patchbay configuration
-
Activate the Patchbay configuration
I haven’t tried it yet for soundcard inputs, but I imagine it would work the same way (just use Output Sockets instead of Input Sockets).