Sorry you had trouble. I have not been able to duplicate this problem on my test server.
Let me help.
What version 4.xx do you have?
The current version contains some improvements to the import routines.
Did you try the button on the Tools tab to "Import forms from 3.xx version"?
Did you ever use the Backup button on version 3.xx? If so, on the Tools tab there is Restore button to restore from that backup.
The upgrade is run automatically only once after installing or upgrading the 4.xx version over a 3.xx versions.
The 4.xx version uses different wp options settings than 3.xx and does not delete the 3.xx settings, they should still be there as long as you do not use the delete button when deactivating the 3.xx plugin.
You can and should always have a backup of your forms.
Read How to backup / restore forms.
Yes you can downgrade to the 3.xx version.
I would like a copy of your 3.xx backup to investigate what happened in case it is something I can fix. You can contact me here if you are able to send it to me.