The TL4 is just slightly larger at the tip opening. The only reason to consider switching is if you feel like you're having to play on harder reeds than what you are confortable with of if you feel like you're getting a more flabby souund than what you want. You can typically play on about a 1/4 to 1/2 strenth reed less on a TL3 compared to a TL4. If you can get to a place to play test, that would be ideal.

Cheers,
     Mark Overton
     www.saxquest.com