The ballots, announced on Tuesday by Russia-installed leaders in Luhansk, Donetsk, Kherson and Zaporizhzhia provinces, are widely seen as a precursor to the illegal annexation of about 15 percent of Ukraine’s territory.
The votes are to be held from Friday to Tuesday. They come after Ukraine launched a successful counteroffensive this month which recaptured large swathes of territory from Russia.
Russia, which announced a partial mobilisation of its army reservists this week, claims the referendums are an opportunity for people in the region to express their view.
“From the very start of the operation ... we said that the peoples of the respective territories should decide their fate, and the whole current situation confirms that they want to be masters of their fate," Russian Foreign Minister Sergei Lavrov said this week.
But it is widely considered Russia intends to use the results to claim popular support for annexation, similar to its takeover of Crimea in 2014.
Putin also hinted this could lead to an escalation of force on Wednesday, saying Russia would “use all the means at our disposal" to protect itself. “This is not a bluff," he said.
World leaders including Joe Biden, UN Secretary-General Antonio Guterres, French President Emmanuel Macron, as well as NATO and the European Union, have all condemned the votes.
The “sham referenda" are “illegal and illegitimate," NATO said.
The Organisation for Security and Co-operation in Europe, which monitors elections, said the outcomes would have no legal force because they do not conform with international law.
Russia already considers Luhansk and Donetsk, which together make up the Donbas region Moscow partially occupied in 2014, to be independent states.
Ukraine and the West consider all parts of Ukraine held by Russian forces to be illegally occupied.
Russia does not fully control any of the four regions, with only around 60% of Donetsk region in Russian hands.
Ukraine said the referendums were a sign Russia was running scared.
“Any decision that the Russian leadership may take changes nothing for Ukraine," Ukraine’s President Volodymyr Zelensky said on Thursday.
“Of interest to us are strictly the tasks before us. This is the liberation of our country, defending our people and mobilising world support (public opinion) to carry out those tasks."