'Converts a list of v3 References to a list of v4 Keys.
Args:
v3_refs: a list of entity_pb.Reference objects
Returns:
a list of entity_v4_pb.Key objects'

def v3_to_v4_keys(self, v3_refs):

    v4_keys = []
    for v3_ref in v3_refs:
        v4_key = entity_v4_pb.Key()
        self.v3_to_v4_key(v3_ref, v4_key)
        v4_keys.append(v4_key)
    return v4_keys
