I've noticed the S4 bluetooth stack acts significantly different than the iPhone.
The metadata is transmitted via AVRCP 1.3+. This has been developed for iPhone for quite awhile and is one of the "selling points" at least for me in an iPhone.
Android/Google has just recently announced higher end development for AVRCP 1.3+ on Android 4.2.2+ but it has not been updated on the stack yet (meaning it's not there and active).
In short... Pandora implemented a software workaround that submits the metadata over bluetooth. This does not exist in Spotify yet (albiet many threads and feature requests by users). The people at XDA have been working on editing the APK, but only do so if you are experienced on your phone:
http://forum.xda-developers.com/showthread.php?t=2081475
Eventually the android OS will do this natively.. But not yet.
Hope this helps!