I've had an S4 for a week now, and I like it a lot. It's a great phone so far. In terms of sound quality through the headphone jack, I am actually pleasantly surprised. It sounds great with my Vsonic GR01, and it's loud enough at half volume and under. I never need anymore than 8 or 9 out of the full 16 steps on the volume control. A volume control with more steps overall wouldn't be a bad thing, but that is nit picking. Everybody complains that the U.S. variants of the S4 don't have a Wolfson DAC, but what I am hearing sounds great. The iPhone 4 didn't have a Wolfson either and it also sounded good. I'm hearing an accurate, full bodied sound with all the details that should be there in the music.
It also sounds great with the aux-in in my car. I will have an apt-x bluetooth receiver in my car tomorrow though, for both music and calls.