The Austin/SanAntonio area is surrounded by the Texas Hill Country, which is where THE BEST BBQ can be found.
Lockhart calls it's self the Capital of Texas BBQ, for good reason. You have Smitty's Market, Blacks BBQ and Kreuz Market, all in one small town. All three are easily in the top ten, IMO.
I also like Coopers in Llano, as well as Louie Mueller Barbecue in Taylor. I also like The Salt Lick, but some people think it is too commercial. I use their rub recipe a lot -- equal parts k-salt, black pepper and cayenne pepper.
There are way too many good places in the Hill Country to hit in four days, especially if you have other things to do.
Just remember, the signature Texas BBQ is brisket, and the Traditional Hill Country brisket is rubbed with salt and pepper only. Get some of that.