I upgraded up to the Galaxy S4 yesterday and was having the same issues as everyone else...no connection via USB. I have mine working just fine...so far. Right now mine is in the process of copying 10+ GB of mp3s with the microSD installed in the phone. Here's what I did...
1) On the phone: Settings>More... (upper right)>About device>Tap "Build Number" 7 times (as suggested earlier in this thread)
2) Enable USB debugging.
3) Unplug and replug phone.
this didn't work for me ...so next thing I did...
1) Install Kies3.
SAMSUNG KIES
2) Unplug cable from USB (I'm using a powered external hub) and reinstall USB drivers (In Kies3... Tools>reinstall device drivers)
3) Reformat microSD while phone unplugged. (Settings > More...>Storage>Format SD card)
If your phone is still plugged in it will be grayed out.
4) Plug phone back into USB.
...and so far it has been copying 10+ GB mp3 files for the past 30 min without errors. According to the "Copying..." dialog I have about 6 hours to go yet. It is slow, but at least it is working. hmm...well I also have Kies3 currently adding music to its library so that could be slowing it down. Won't know until I do further tests without it running.
I do have a USB external reader for microSD but I don't want to have to open up the phone every time I want to load up some music.
Would be better if it could transfer as fast as iTunes does to my Nano. The Nano is the only Apple thing I own. ...and I only bought it because of my car stereo...just plug it in, shuffle all

The Android phone I had at the time (LG Optimus C) kept randomly crashing whether I was using it or not....so it was no good for music in the car.
Anywho, I hope this info helps someone.
