'Migrate a v3 JSON configuration to v4.
:param bytes config: The v3 JSON data.
:return bytes: The v4 JSON data.'

@classmethod
def upgrade_from_v3(cls, config):

    decoded_config = loads(config)
    decoded_config[u'version'] = 4
    decoded_config[u'deployment'][u'persistent_state'] = {_CLASS_MARKER: u'PersistentState', u'blockdevice_ownership': {u'values': [], _CLASS_MARKER: 'PMap'}}
    return dumps(decoded_config)
