code: func (a Address) To4() Address {
  	const (
  		ipv4len = 4
  		ipv6len = 16
  	)
  	if len(a) == ipv4len {
  		return a
  	}
  	if len(a) == ipv6len &&
  		isZeros(a[0:10]) &&
  		a[10] == 0xff &&
  		a[11] == 0xff {
  		return a[12:16]
  	}
  	return ""
  }
docs: // To4 converts the IPv4 address to a 4-byte representation.
  // If the address is not an IPv4 address, To4 returns "".