Illinois and Connecticut have joined New York and California in telling residents to stay home during the coronavirus pandemic.
The move means around 75 million Americans are affected by the strict “social distancing” measures, which are intended to stop the spread of the disease.
Residents of the four states must stay inside except for trips to grocery stores, pharmacies, petrol stations and other “essential businesses”. Most workplaces are also closed.
Illinois Governor JB Pritzker said the rules were to “avoid the loss of tens of thousands of lives”, as the state – which includes the city of Chicago – registered 585 cases of the virus.
The four states account for 31% of the US economy.
‘Post-apocalyptic future’ as Los Angeles goes quiet
New Jersey’s governor Phil Murphy has said he plans to issue a similar order in the next 24 hours.
Earlier in the week, California’s state governor Gavin Newsom issued a state-wide “stay at home” order, which took effect from Thursday evening and will remain in place until further notice.
More from Covid-19
Trump congratulates Kim Jong Un on managing coronavirus in North Korea
Coronavirus: 4,500 retired doctors and nurses sign up to battle COVID-19 pandemic
Man charged in UK with trying to sell fake coronavirus treatment kits around world
Coronavirus: Boris Johnson urges adults to stay away from elderly mums on Mother’s Day
Coronavirus: Two NHS medics on ventilators after contracting COVID-19 from patients
Coronavirus: 8,000 more hospital beds and 20,000 extra staff join COVID-19 fight
He said the state needed to “recognise the reality” and “make tough decisions”, although conceded that home isolation was not his “preferred choice”.
‘They call it the apocalypse’
Inside Italy’s hospital hardest-hit by coronavirus
New York governor Andrew Cuomo also issued an executive order to state residents, meaning all non-essential workers must stay at home or cease working completely.
Mr Cuomo added that the over-70s, or those with underlying health conditions, must stay at home, apart from solo exercising.
The states of New York and California have been some of the hardest hit in the US, with combined confirmed cases of COVID-19 at more than 6,500, with 57 people having lost their lives.
California’s governor said modelling has shown 56% of California residents are expected to contract COVID-19 over the next eight weeks, requiring nearly 20,000 more hospital beds than the state can currently provide.
:: Listen to Divided States on Apple podcasts, Google Podcasts, Spotify, and Spreaker
He also warned infection rates are doubling every four days in some parts of the state.
Los Angeles, the nation’s second-largest city, is likely to be “disproportionately impacted” in the coming weeks, Mr Newsom added.