The first two doses of Td are valid because they are separated by at least 4 weeks. However, the minimum interval between the second and third doses of tetanus-containing vaccine is 6 calendar months. So, the Td component of the Tdap dose is not valid because it was given only 4 months after the second dose. The pertussis component can be counted as valid. The patient should receive another dose of Td or Tdap 6 months after the invalid Tdap dose.