You need to be detained under section 4 if:
You have a mental disorder
it is urgently necessary for you to be admitted to hospital and detained, and
waiting for a second doctor to confirm that you need to be admitted to hospital on a section 2 would cause "undesirable delay"
You can be sectioned by one doctor only - together with the approved mental health professional - anf you can be taken to hospital in an emergency and assessed there.Your rights are diffrent compared to your rights under other sections. For example, you cannot be treated without your consent. You can be detained for upto 72 hours under this section.