All my past saved sessions are gone as well too. It's a devasting amount of data that was lost.
For reference, when the update went live on the 12th, I tried the "repair extension", option in Chrome which apparently reset and forced Session Buddy to update.
It seems like normally it's supposed to convert all your saved sessions to be compatible with V4 but this never happened. I hope the sessions are still in the SB data file just not readable and not deleted and that there's some way to convert them to be compatible with the new version.