Googling the error message text yields some hits on this forum:
Those posts involve different releases of SQL Developer, one an Early Adopter build of 4.0, the other 4.0.2. Perhaps something there applies to your case.
When you post please provide your full sql developer version.
If you aren't using the latest version (220.127.116.11.21) try downloading and using it. There have been a lot of improvements in the latest version.
It worked after i specified my connection name to both the parameters repo & db like this...
C:\sqldeveloper\sqldeveloper\bin>sdcli unittest -run -suite -name TC_01_Test_Gehend -repo Legato_Test -db Legato_Test