On my old phone (Droid 3) I used Go SMS Pro (https://play.google.com/store/apps/details?id=com.jb.gosms
). It has an option to have a pop up of a recieved text. you can even reply without opening the messaging app.
Note: I have not tried it on my S4 yet. I am sure it works though. Its a well devolved app.
Other messaging apps might have the same option (i.e. champ sms, handcent sms)