I just got my Galaxy S4 and when I tried to use it with unity it didn’t work… After 2H I finally figured it out! Here’s how!
First of all I just wanna say that I’ve been using unity with android for more than a year
Ok let’s start!
1: Install Android SDK and check the latest android version + “Google USB Driver”
2: We need to enable USB debugging! By default the developer options on Jelly Bean (4.2.2) are hidden,
tap on your “Build Number” five times, and you should see a toast notification pop up saying “Developer mode has been enabled.”
Voilà! Back out of the About Device menu and you’ll see Dev Options back where it belongs, so you can turn on USB Debugging
Now close Unity and connect your phone to your computer via USB
3: Press play and if the unity logo + text disappears, congrats!
If not let’s continue on!
4: Download “Kies” from samsung, which is like a iTunes knockoff but whatever it installs drivers 4 u
http://www.samsung.com/us/kies/
Now go back to step 3!
5: Uninstall Bluestacks (If you don’t know what it is, skip this), it acts like a real device so unity ignores your phone! Then do step 3
6: Nothing above worked?
Search for “cmd.exe” on your comp, then left click and copy it, then go to where u installed your android sdk and place it in platform-tools
then run it and type:
adb devices
If your phone is not listed then try to switch USB port or maybe even try another micro USB cable, but if it is listed go to step 3!
That’s it! If it worked please reply! 
If you’re feeling a little bit extra generous today and a dollar or two laying around you can donate to me just private message me 