My S4's favorite tricks are making screen shots and changing my phone setting from noise to vibrate (so I don't hear when the phone rings).
Today I had a really odd experience. I was in my car and listening to an audio book. I texted my boss a response. Later I looked at the phone and saw an odd post and thought he had accidentally posted something to me about some guy that something was going to happen to and I was trying to text him back "I think you sent me something by accident" but I couldn't find the reply box and couldn't find a delete button to delete the mis-sent message. I finally realized what had happened. I had used the voice feature to text him and after I sent the message, it had kept translating my audio book into a text message. Really glad I hadn't accidentally sent that to *him* -- he would have thought I'd popped my cork for sure.
Anyway, I know none of this helps you particularly, but the S4 does seem to have a mind of its own and it drives me nuts. I think it has launched music once or twice, but not persistently. My biggest pet peeve is setting it back to the ringtone after I have had it on vibrate and it keeps turning itself back to vibrate. I had to turn the phone off and on again the other day before it would leave it on the ringtone.
There *is* a setting somewhere that makes it not necessary to actually touch the screen to launch things; you might want to check through your settings and look for that. Maybe that's why it starts playing when you walk past it.
I love my S4 but these things drive me nuts because I have missed a lot of calls because the phone didn't ring when it should have because the phone muted itself and I didn't feel it vibrate.
Time to build a bigger bridge.