On SFOS 4.0, default user would depend on whether you've got to 4.0 via OTA or fresh install. That should be for all official and on ports as Xperia Tama. As a result: if you flash new 4.0 image -> defaultuser is used if you come from earlier versions -> nemo is kept. I don't remember when the user name switch happened. I have a feeling it was earlier than 4.0