def assemble_content_aad(message_id, aad_content_string, seq_num, length):
    """Assembles the Body AAD string for a message body structure.

    :param message_id: Message ID
    :type message_id: str
    :param aad_content_string: ContentAADString object for frame type
    :type aad_content_string: aws_encryption_sdk.identifiers.ContentAADString
    :param seq_num: Sequence number of frame
    :type seq_num: int
    :param length: Content Length
    :type length: int
    :returns: Properly formatted AAD bytes for message body structure.
    :rtype: bytes
    :raises SerializationError: if aad_content_string is not known
    """
    if not isinstance(aad_content_string, aws_encryption_sdk.identifiers.ContentAADString):
        raise SerializationError("Unknown aad_content_string")
    fmt = ">16s{}sIQ".format(len(aad_content_string.value))
    return struct.pack(fmt, message_id, aad_content_string.value, seq_num, length)