Together the states represent about 35 per cent of Bolivia's more than 8.5 million people.
 
A 130-member assembly in Santa Cruz, an opposition stronghold and Bolivia's wealthiest state, earlier approved an "autonomy statute" under which the state would hold on to nearly two-thirds of its tax revenues and consider establishing its own police force.
 
Local leaders have promised a referendum in the coming months to approve the measures and the other three states have said they plan to table similar statutes.
 
Reinforcements
 
The government sent 400 police reinforcements to Santa Cruz this week, but denied sending soldiers.
 
San Miguel said the government had not mobilised "a military detachment in the city of Santa Cruz".
 
Juan Carlos Urenda, a Santa Cruz leader, said the proposed autonomy statute was not an act of secession as it recognised Santa Cruz as part of Bolivia, with the central government continuing to maintain control over foreign affairs, defence and the currency.
 
Alvaro Garcia, the country's vice-president, appeared on national television on Thursday to address leaders of the four states, saying he feared "a catastrophic standoff" had been reached.
 
Rich and poor
 
The four states object to a constitutional overhaul planned by Evo Morales, the Bolivian president, which would increase his presidential powers, improve the rights of Bolivia's indigenous majority and redistribute wealth from their low-lying richer areas to the poorer heights of the Andes.
 
In November, the country's assembly approved a draft constitution that would establish a multi-ethnic state with self-governing regions for indigenous groups.
 
But the largest opposition party boycotted the assembly and only 153 of the assembly's 255 delegates were present at the vote.
 
The charter, which Bolivians must approve in referendums due next year, would shift more power to central authorities at the expense of the country's nine states.
 
Urenda described the constitution as a racist trick that would allow indigenous communities to violate the states' territorial integrity.
 
Morales's core support, which secured him his December 2005 election victory, comes from Bolivia's poor, indigenous majority that lives primarily in the arid Andean highlands.