Australia Government Dept of Health and Aged Care
"For most people, a primary vaccination course consists of 2 doses.
A third primary dose is recommended for people aged 6 months or older with severe immunocompromise."
"A second booster dose (that is a 4th dose) is recommended for people in the following groups, 3 months after the first booster dose:
people 50 years or older
residents aged 16 years and older of an aged care or disability care facility
people aged 16 years and older who have complex, chronic, or severe medical conditions that increase their risk of severe illness from COVID-19
people aged 16 years and older with disability with significant or complex health needs, or multiple comorbidities that increase the risk of poor outcome from COVID-19.
People aged 30 to 49 years can receive a second booster dose of a COVID-19 vaccine, but the benefit for people in this age group is less certain. ATAGI encourages people in this age group to have a discussion with their regular medical provider to review their individual health needs and the benefits and risks of a second booster dose.
"A second booster dose (that is a 4th dose) is recommended for people in the following groups, 3 months after the first booster dose:
people 50 years or older
residents aged 16 years and older of an aged care or disability care facility
people aged 16 years and older who have complex, chronic, or severe medical conditions that increase their risk of severe illness from COVID-19
people aged 16 years and older with disability with significant or complex health needs, or multiple comorbidities that increase the risk of poor outcome from COVID-19."
https://www.health.gov.au/initiatives-and-programs/covid-19-vaccines/advice-for-providers/clinical-guidance/clinical-recommendations#booster-dose-recommendations-