Joel,
If I had to guess, I think it is because of your importing preferences from a newer version of PM to an older version of PM. Like I said earlier, this has not been tested (to my knowledge you're the first person to try it) and is not supported at this time.
You can certainly go and selectively remove various preferences data until it begins to work for you. Connection Settings may be the problem, but it may be the overall template settings that are the problem (PM4 didn't have some of the features PM5 does, like the JPEG sizing options).
The uploader template settings are in the following path:
<your PM prefs folder>\fileupload\templates\builtin\
There should be a FTP.yml file and possibly a FTP-conn.yml file. You may need to delete both of them. You could rename them or move them to a different folder instead of deleting.
-Kirk