If you're in SA on the 4th, there will be lots of fireworks displays to choose from. SeaWorld, Woodlawn Lake, Riverwalk, probably Schlitterbahn.
I always like to go to small towns for the 4th, if I can, and see the town parade and whatever else there is. That's a good area to explore. My ex and I won a horseshoe tournament once on the 4th of July near Canyon, at a little celebration put on by volunteer firefighters which we stumbled upon while driving around.
If you really are going to hdqr in SA, I'd try Boerne or Seguin or New Braunfels, depending on which side of town you'll be on and how far you want to drive. Also, some of the AFBs in the area do fireworks, IIRC.
Wherever they are, they will be very closely controlled because of our major drought and fire weather, so be careful.
Have fun and enjoy Texas!