Our best guess is that SDG-4 is down. There was one final report from Tennessee at around 00:00 UTC, but the signal was weak and fading rapidly. No further reports have been received.
I have contacted some hams in North Carolina via Echolink - none could hear the beacon.
Two possible scenarios come to mind... First, the balloon simply may not have withstood the peak solar flux (around 5pm local time) and popped. This is entirely possible. Second, the balloon may have encountered terrain over the mountains - i.e., it might have crashed into a tree! I think this is less likely - HYSPLIT showed altitude above ground level dropping to 1000 meters, but never lower. On the other hand, the Google Earth output from HYSPLIT did, in fact, show the trajectory going right through the side of a mountain (and emerging on the other side!).
For now, we are calling the search off... No more blind emails sent off to hams-unknown in the southeast, no more Echolink calls.
If anyone has any further news on SDG-4 beyond what has been posted here already, please let us know!