Victorian Acting Premier James Merlino said all four new cases were linked to the outbreak.
"Which is, again, a good thing," he said.
There are currently 15,000 primary and secondary contacts connected to the Melbourne cluster, with this number only tipped to rise.
More than 120 public exposure sites have been linked to the outbreak to date.
People are urged to regularly check the Victorian coronavirus website throughout the day, as it's expected the four new cases will result in more exposure sites.
Until June 3 at 11.59 pm, there will be just five reasons to leave home.
These include shopping for necessary goods and services, authorised work or permitted education, exercise, caregiving for compassionate and medical reasons and to get vaccinated, with over-40s eligible from this morning.
People will need to stay within 5 km of their home for shopping and exercise, unless the nearest shops are further away.
Masks must be worn both indoors and outdoors unless an exemption applies.
There are to be no visitors to homes other than intimate partners, but single bubbles will be permitted.
There will also be no public gatherings, with restaurants, pubs, and cafes to provide takeaway only.
Childcare and kinders will open as per the circuit-breaker lockdown earlier this year, while schools will move to remote learning, except for vulnerable children and children of authorised workers.
Higher education will move to remote learning only.
Approved professional sporting events will proceed but without crowds.
Visits will not be allowed at aged care facilities except for limited reasons, and hospital visits will only be permitted for end-of-life reasons, to support a partner during birth, or for a parent accompanying a child.
Funerals will be capped at 10 people – plus those running the service – and weddings cannot proceed unless for end-of-life or deportation reasons.
Religious activities will not proceed other than through broadcast, with a maximum of five people allowed.
Mr Merlino said this circuit-breaker was necessary to contain the spread of the "quick and contagious" Indian variant of the virus.
"Unless something drastic happens, this will become increasingly uncontrollable," he said.
When asked why a state-wide lockdown was necessary when all positive cases were in Melbourne, Victorian chief health officer Brett Sutton said it was to prevent the virus from spreading to regional areas.
"I understand the reflections of regional Victorians who are rightly saying they have not had cases for many months — for some local government areas, for over a year," he said.
"The reality is if we carved out a part of regional Victoria and said 'no restrictions in that place', that's where everyone from Melbourne would go.
"And that's when everyone would introduce this virus into previously unaffected areas. There are operational considerations about how you stop some of the potential movement."
Victoria also recorded two new cases in hotel quarantine yesterday.
More than 17,000 vaccine doses were administered yesterday, with 47,462 test results received.
