def get_special_tokens(vocab_size):
  """Gets the ids of the four special tokens.

  The four special tokens are:
    pad: padding token
    oov: out of vocabulary
    bos: begin of sentence
    eos: end of sentence

  Args:
    vocab_size: The vocabulary size.

  Returns:
    The four-tuple (pad, oov, bos, eos).
  """
  pad = 0
  oov = vocab_size + 1
  bos = vocab_size + 2
  eos = vocab_size + 3

  return pad, oov, bos, eos