Could you build 2 SIP Peer profiles?
One with 3 trunks and one with 4 trunks with the same registration information.
In ARS point your outbound to the profile with 3 trunks.
Might work
That should work, depending on your provider. A nicer option might be to make a loopback ip trunk. So an ip trunk towards the controller itself. You can limit the number of trunks on this. You can route your outbound traffic through this. If all channels are used, and you want it to switch to your 2nd route, you should use a route list with the loopback trunk 1st and 2nd isdn.
You would need ip networking/enterprise lic for this to work....