The first Galaxy S4 sets to receive the Stagefright fixer update have become that of AT&T as an update for AT&T S4 devices has started rolling out today, while the other S4 sets the world over should be receiving the same update — with different build no. of course — soon enough. An update is also being pushed for the AT&T S4 Active, btw. Samsung’s been very busy rolling out the Stagefright fix for its device, and nearly all Galaxy S6 sets have received it so far, while most of the S5 and Note 4 (and Edge) sets have gotten the critical bug fixer update too.
The build no. of the Stagefright fixer OTA update is I337UCUGOC4 for AT&T Galayx S4 (model no. SGH-i337) while that for S4 Active is I537UCUCOC7 (model no. SGH-i537). To force the OTA update on your device, go to Settings > About device > Software update, and then tap on ‘check for update’ button.