I would imagine that it is possible. You can play multiple soundtracks simultaneously. If you are using Minim, I imagine that the 30 second track would be an AudioSnippet and the 4 second track would be an AudioSample. You would then do something like:

1. int started;
2. boolean done;
   
3. <AudioSample>.play();
4. started = millis();

and then:

1. if(millis() - started > 11000 && !done) {
2.       <AudioSample>.trigger();
3.       done = true;
4. }

Of course you would insert variable names in the <>, but you get the idea.