code: def as4_capability(self, **kwargs):
          
          enabled = kwargs.pop('enabled', True)
          callback = kwargs.pop('callback', self._callback)
  
          if not isinstance(enabled, bool):
              raise ValueError('%s must be `True` or `False`.' % repr(enabled))
  
          as4_capability_args = dict(vrf_name=kwargs.pop('vrf', 'default'),
                                     rbridge_id=kwargs.pop('rbridge_id', '1'))
  
          as4_capability = getattr(self._rbridge,
                                   'rbridge_id_router_router_bgp_router_bgp'
                                   '_attributes_capability_as4_enable')
  
          config = as4_capability(**as4_capability_args)
  
          if not enabled:
              capability = config.find('.//*capability')
              capability.set('operation', 'delete')
              # shutdown = capability.find('.//*as4-enable')
              # shutdown.set('operation', 'delete')
  
          return callback(config)
docs: Set Spanning Tree state.
  
          Args:
              enabled (bool): Is AS4 Capability enabled? (True, False)
              callback (function): A function executed upon completion of the
                  method.  The only parameter passed to `callback` will be the
                  ``ElementTree`` `config`.
  
          Returns:
              Return value of `callback`.
  
          Raises:
  
              ValueError: if `enabled` are invalid.
  
          Examples:
              >>> import pynos.device
              >>> switches = ['10.24.39.211', '10.24.39.203']
              >>> auth = ('admin', 'password')
              >>> for switch in switches:
              ...     conn = (switch, '22')
              ...     with pynos.device.Device(conn=conn, auth=auth) as dev:
              ...         output = dev.bgp.local_asn(local_as='65535',
              ...         rbridge_id='225')
              ...         output = dev.bgp.as4_capability(
              ...         rbridge_id='225', enabled=True)
              ...         output = dev.bgp.as4_capability(
              ...         rbridge_id='225', enabled=False)