I heard by double-tapping the home button, the S4 will launch S-Voice. But when I tap the home button twice, nothing happens. It's there a setting I have to turn on to get this to work?
Mine works and I don't remember having to make any changes that would have changed it one way or the other.
It may depend on the carrier. My Verizon S4 didn't default to the double tap to launch.
ETA.... I'm using Nova Launcher, so that could be the reason it doesn't launch with the double tap. What launcher are you using, OP?