So when I receive text messages on my Galaxy S4, why doesn't the notification sound on my car speakers like it does on my Prius where I can reply and read text I received ? Right now only voice calls get picked up by 5000NEX...

For iPhones you can use Siri Eyes Free to respond via the NEX headunit. I am not sure how the Androids are handled, if at all in a similar fashion. My Prius also has its handy text prompts on the screen, or to be read out loud, but its clunky and somewhat uninstuitive which can lead to wandering eyes while driving. I think Pioneers approach is to mirror Apples, which is to move everything to voice command and stop showing texts on screens to keep our eyes on the road. Once Android incorperates a similar technology I bet Pioneer will enable it.