The mayor’s Russian oligarch “problem” just got bigger.

Russian steel czar Roman Abramovich has tightened his grip on East 75th Street, scooping up a total of now four townhomes in a row, The Post has learned. The details of Abramovich’s fourth acquisition come just days after Mayor de Blasio griped about wealthy Russians using “ill-gotten gains” to snap up US property.

Abramovich, who is worth an estimated $9.6 billion, secretly purchased the home at 9 E. 75th St. as part of his collection, which also includes 11, 13 and 15 E. 75th St.

He had initially hoped to combine Nos. 11, 13 and 15 to form one of the largest homes in the city. But then No 9 became available for sale.

Abramovich now believes that his proposed new combo of 9, 11 and 13 will be a better “fit” — and still give him a mansion fit for a king, a source said .

“It will look better and bring a more balanced facade to the street — it will have more symmetry,” said a source close to architect Stephen Wang, who is working with the Swiss architecture firm Herzog & de Meuron as a “design collaborator” to create Abramovich’s sprawling home.

It’s unclear what Abramovich, the owner of the Chelsea Football Club soccer team in Britain, plans to do with 15 E. 75th St.

If he were to later try to combine all four properties, he would have the biggest home the city has ever seen — much to de Blasio’s ire.

Earlier this month, the mayor said Russian oligarchs “basically stole the wealth of their country with the help of their government” — and then infiltrated the city to set up their home base.

“It manifests here as people with a lot of ill-gotten gains buying a lot of property,” de Blasio said of the “problem.”

“I don’t like it one bit. I wish I had a specific law or approach to address it. I haven’t found one yet. But, of course, it’s distressing,” de Blasio said.

Abramovich, who allegedly earned his fortune in rigged privatization auctions, paid a total of $96 million total for all four properties which were purchased using LLCs, according to city property records.

“If the sellers had known he was buying up the block, they could have held out for higher prices,” a source noted.

Abramovich just split from his wife, Dasha Zhukova, a close friend of Ivanka Trump and the daughter of another oligarch. Zhukova is the mother of two of Abramovich’s seven children.

Abramovich and his former business partner, Boris Berezovsky — who was found dead in 2013 — were both credited with helping to bring Vladimir Putin to power in 2000.