I too am having a group messaging problem with my S4. I am able to send single SMS messages but as soon as I add a second person, the MMS just sits on the screen saying "SENDING".. And won't go even after hours of writing. It doesn't seem to matter whether I use Mobile Data ON or OFF.
I follow my data usage closely as I have a limited data plan and for the past few weeks there has been no data moving over the phone line and only a few mb's through wifi. In particular, my calendar has stopped syncing with my Google online calendar..
So it appears that for some weird reason, my phone has stopped sending or receiving data in most forms. And it appears to have happened roughly around the time that your messaging got screwed up too. However, one app that is a reminder list (to-do lists, etc) still syncs with it's online site. So it is able to send and receive data. ??
I have spent a couple of hours during the past few days with the support people at Consumer Cellular and have changed/edited/reviewed virtually every available setting re: MMS and APN's, etc. to no avail. This group messaging problem occurred approximately 8 months ago and at that time the support person at Consumer Cell was able to fix it in about 20 minutes.
Any ideas out there??
Is this phone ready for the scrap pile?