Pretty sure the Thermal is dual tip for the 5th gen only.
I was (am) in the same situation as you. Here's a thread that I posted looking for answers:
4th Gen Dual Tip Catback Options
Basically, the options were custom, 5th gen thermal adapted to the 4th gen, or Brullen (which they pretty much make a custom for you upon ordreing).
I ended up going with some custom piping and a magnaflow dual tip muffler. Haven't had a chance to put it on yet, but the muffler looks good sitting in my room!