Section 4 of the Mental Health Act is an emergency application for detention in hospital for up to 72 hours. It requires only one medical recommendation from a doctor and the application is usually by an Approved Mental Health Professional, on very rare occasions it can be applied by the Nearest Relative.
For Section 4 you need the applicant which is usually the Approved Mental Health Professional or Nearest Relative and one doctor, preferably the doctor who knows the service user such as their GP or known Psychiatrist. Otherwise it would be a section 12 approved doctor.
Section 4 can last up to 72 hours but with another doctor providing the second medical recommendation this can be converted to Section 2. There are no rights to appeal against Section 4 but you can appeal against Section 2.
Section 4 is rarely used as it is not good practice to admit someone to hospital with only one medical recommendation, but occasionally it is necessary due to the urgency of the situation.