Ensure that you are using the same character set in Database and SQL Developer. You can set the characterset in SQL Developer Preferences.
At SQL Developer Preferences (Database > NLS), there are definitions for Language, Territory but not for Character Set.
I experienced the same problem as you but cannot find any solution yet.
My DB characterset is KO16MSWIN949 and SQL developer Encoding setting is MS949.
A column default value is set as 'EA' and this seems to give error.
SQL Developer 3.2.20.09 version does not make erroe message with the same setting.
It seems to be a bug of SQL Developer version 4.
Try the new version of SQL Developer (220.127.116.11.48), the problem seems to have disappeared.