So that numbering will also be a priority list? I don't know any product that does that, but you can probably make your own circuit. I can help you with schematics, but I'm in the middle of midterms right now so all I have time for right now is to offer some pointers. For example, at the bottom of this page is an interesting way to detect if there is audio and then do something with it: http://www.doctronics.co.uk/4016.htm. Have a couple of these and a simple truth table, and you've got your 4 way audio switch with priority switching.